diff --git a/src/libraries/System.Diagnostics.DiagnosticSource/tests/DiagnosticSourceTests.cs b/src/libraries/System.Diagnostics.DiagnosticSource/tests/DiagnosticSourceTests.cs
index ae93b7a2e4f761..a806cc0a018fb6 100644
--- a/src/libraries/System.Diagnostics.DiagnosticSource/tests/DiagnosticSourceTests.cs
+++ b/src/libraries/System.Diagnostics.DiagnosticSource/tests/DiagnosticSourceTests.cs
@@ -534,6 +534,7 @@ public void AllListenersCheckCatchupList()
         [InlineData(100, 102)]
         [InlineData(100, 103)]
         [InlineData(100, 104)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/79906", TestRuntimes.Mono)]
         public void AllSubscriberStress(int numThreads, int numListenersPerThread)
         {
             // No listeners have been created yet
diff --git a/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.Windows.cs b/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.Windows.cs
index 57dff3dce3b024..14556b807fb67a 100644
--- a/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.Windows.cs
+++ b/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.Windows.cs
@@ -16,6 +16,7 @@ private static bool IsAdmin_IsNotNano_RemoteExecutorIsSupported_CanShareFiles
         [ConditionalFact(nameof(IsAdmin_IsNotNano_RemoteExecutorIsSupported_CanShareFiles))] // Nano has no "netapi32.dll", Admin rights are required
         [PlatformSpecific(TestPlatforms.Windows)]
         [OuterLoop("Requires admin privileges")]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/80019", TestRuntimes.Mono)]
         public void TestUserNetworkCredentialsPropertiesOnWindows()
         {
             const string ShareName = "testForDotNet";
diff --git a/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.cs b/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.cs
index 686780fd018d72..3abc9b6e71aa8d 100644
--- a/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.cs
+++ b/src/libraries/System.Diagnostics.Process/tests/ProcessStartInfoTests.cs
@@ -465,6 +465,7 @@ public void TestWorkingDirectoryPropertyInChildProcess()
         [ConditionalFact(nameof(IsAdmin_IsNotNano_RemoteExecutorIsSupported))] // Nano has no "netapi32.dll", Admin rights are required
         [PlatformSpecific(TestPlatforms.Windows)]
         [OuterLoop("Requires admin privileges")]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/80019", TestRuntimes.Mono)]
         public void TestUserCredentialsPropertiesOnWindows()
         {
             using Process longRunning = CreateProcessLong();
@@ -1060,6 +1061,7 @@ public void StartInfo_WebPage()
         [MemberData(nameof(UseShellExecute))]
         [OuterLoop("Launches notepad")]
         [PlatformSpecific(TestPlatforms.Windows)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34685", TestRuntimes.Mono)]
         public void StartInfo_NotepadWithContent(bool useShellExecute)
         {
             string tempFile = GetTestFilePath() + ".txt";
@@ -1093,6 +1095,7 @@ public void StartInfo_NotepadWithContent(bool useShellExecute)
                                                     nameof(PlatformDetection.IsNotWindows8x))] // https://github.com/dotnet/runtime/issues/22007
         [OuterLoop("Launches notepad")]
         [PlatformSpecific(TestPlatforms.Windows)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34685", TestRuntimes.Mono)]
         public void StartInfo_TextFile_ShellExecute()
         {
             // create a new extension that nobody else should be using
@@ -1272,6 +1275,7 @@ public void InitializeWithArgumentList()
         [MemberData(nameof(UseShellExecute))]
         [OuterLoop("Launches notepad")]
         [PlatformSpecific(TestPlatforms.Windows)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34685", TestRuntimes.Mono)]
         public void StartInfo_NotepadWithContent_withArgumentList(bool useShellExecute)
         {
             string tempFile = GetTestFilePath() + ".txt";
diff --git a/src/libraries/System.Diagnostics.Process/tests/ProcessTests.Unix.cs b/src/libraries/System.Diagnostics.Process/tests/ProcessTests.Unix.cs
index b29de573446e77..2085fddbb5d202 100644
--- a/src/libraries/System.Diagnostics.Process/tests/ProcessTests.Unix.cs
+++ b/src/libraries/System.Diagnostics.Process/tests/ProcessTests.Unix.cs
@@ -81,6 +81,7 @@ public void ProcessStart_UseShellExecute_OnLinux_ThrowsIfNoProgramInstalled()
         [Fact]
         [OuterLoop("Opens program")]
         [SkipOnPlatform(TestPlatforms.MacCatalyst, "In App Sandbox mode, the process doesn't have read access to the binary.")]
+        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.Android | TestPlatforms.Browser, "Not supported on iOS/tvOS/Android/Browser.")]
         public void ProcessStart_DirectoryNameInCurDirectorySameAsFileNameInExecDirectory_Success()
         {
             string fileToOpen = "dotnet";
@@ -110,6 +111,7 @@ public void ProcessStart_DirectoryNameInCurDirectorySameAsFileNameInExecDirector
 
         [Fact]
         [OuterLoop]
+        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.Android | TestPlatforms.Browser, "Not supported on iOS/tvOS/Android/Browser.")]
         public void ProcessStart_UseShellExecute_OnUnix_OpenMissingFile_DoesNotThrow()
         {
             if (OperatingSystem.IsLinux() &&
@@ -129,6 +131,7 @@ public void ProcessStart_UseShellExecute_OnUnix_OpenMissingFile_DoesNotThrow()
 
         [Theory, InlineData(true), InlineData(false)]
         [OuterLoop("Opens program")]
+        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.Android | TestPlatforms.Browser, "Not supported on iOS/tvOS/Android/Browser.")]
         public void ProcessStart_UseShellExecute_OnUnix_SuccessWhenProgramInstalled(bool isFolder)
         {
             string programToOpen = s_allowedProgramsToRun.FirstOrDefault(program => IsProgramInstalled(program));
diff --git a/src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs b/src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs
index a102fee93075a5..bbb0b523cc900e 100644
--- a/src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs
+++ b/src/libraries/System.Diagnostics.Process/tests/ProcessTests.cs
@@ -244,7 +244,7 @@ public void TestStartWithBadWorkingDirectory()
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.HasWindowsShell))]
         [OuterLoop("Launches File Explorer")]
-        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS, "Not supported on iOS and tvOS.")]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34685", TestRuntimes.Mono)]
         public void ProcessStart_UseShellExecute_OnWindows_OpenMissingFile_Throws()
         {
             string fileToOpen = Path.Combine(Environment.CurrentDirectory, "_no_such_file.TXT");
@@ -257,7 +257,7 @@ public void ProcessStart_UseShellExecute_OnWindows_OpenMissingFile_Throws()
         [InlineData(true)]
         [InlineData(false)]
         [OuterLoop("Launches File Explorer")]
-        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS, "Not supported on iOS and tvOS.")]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34685", TestRuntimes.Mono)]
         public void ProcessStart_UseShellExecute_OnWindows_DoesNotThrow(bool isFolder)
         {
             string fileToOpen;
diff --git a/src/libraries/System.DirectoryServices.AccountManagement/tests/PrincipalContextTests.cs b/src/libraries/System.DirectoryServices.AccountManagement/tests/PrincipalContextTests.cs
index c011aa7e8ecc5e..29fe1ba217dfc6 100644
--- a/src/libraries/System.DirectoryServices.AccountManagement/tests/PrincipalContextTests.cs
+++ b/src/libraries/System.DirectoryServices.AccountManagement/tests/PrincipalContextTests.cs
@@ -24,6 +24,7 @@ public void Ctor_ContextType()
         }
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoNorServerCore))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34442", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop("Takes too long on domain joined machines")]
         [InlineData(ContextType.Machine, null)]
         [InlineData(ContextType.Machine, "")]
@@ -50,6 +51,7 @@ public void Ctor_ContextType_Name(ContextType contextType, string name)
         }
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoNorServerCore))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34442", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop("Takes too long on domain joined machines")]
         [InlineData(ContextType.Machine, null, null)]
         [InlineData(ContextType.Machine, "", null)]
@@ -76,6 +78,7 @@ public void Ctor_ContextType_Name_Container(ContextType contextType, string name
         }
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoNorServerCore))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34442", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop("Takes too long on domain joined machines")]
         [InlineData(ContextType.Machine, null, null, ContextOptions.Negotiate)]
         [InlineData(ContextType.Machine, "", null, ContextOptions.Negotiate)]
diff --git a/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ActiveDirectoryInterSiteTransportTests.cs b/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ActiveDirectoryInterSiteTransportTests.cs
index 7d99c8637e21b0..be454aa9c34728 100644
--- a/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ActiveDirectoryInterSiteTransportTests.cs
+++ b/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ActiveDirectoryInterSiteTransportTests.cs
@@ -48,6 +48,7 @@ public void FindByTransportType_DomainNoDomainAssociatedWithoutName_ThrowsArgume
         }
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoServer))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34442", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop("Takes too long on domain joined machines")]
         [InlineData(DirectoryContextType.ApplicationPartition)]
         [InlineData(DirectoryContextType.DirectoryServer)]
diff --git a/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/DomainControllerTests.cs b/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/DomainControllerTests.cs
index e832acc96a4581..b69b52ca9fa269 100644
--- a/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/DomainControllerTests.cs
+++ b/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/DomainControllerTests.cs
@@ -27,6 +27,7 @@ public void GetDomainController_InvalidContextType_ThrowsArgumentException(Direc
         }
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoServer))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34442", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop("Takes too long on domain joined machines")]
         [InlineData("\0")]
         [InlineData("[")]
diff --git a/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ForestTests.cs b/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ForestTests.cs
index 64e7079fc0036b..c32dd0c989ea69 100644
--- a/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ForestTests.cs
+++ b/src/libraries/System.DirectoryServices/tests/System/DirectoryServices/ActiveDirectory/ForestTests.cs
@@ -48,6 +48,7 @@ public void GetForest_NonNullNameAndNotRootedDomain_ThrowsActiveDirectoryObjectN
         }
 
         [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsNotWindowsNanoServer))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34442", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop("Takes too long on domain joined machines")]
         [InlineData(DirectoryContextType.Forest, "\0")]
         [InlineData(DirectoryContextType.DirectoryServer, "server:port")]
diff --git a/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntry.LongFile.Tests.cs b/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntry.LongFile.Tests.cs
index 613e30c541d1c4..1368f452c01652 100644
--- a/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntry.LongFile.Tests.cs
+++ b/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntry.LongFile.Tests.cs
@@ -27,6 +27,7 @@ public static IEnumerable<object[]> WriteEntry_LongFileSize_TheoryData()
 
         [Theory]
         [MemberData(nameof(WriteEntry_LongFileSize_TheoryData))]
+        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.Android | TestPlatforms.Browser, "Needs too much disk space.")]
         public void WriteEntry_LongFileSize(TarEntryFormat entryFormat, long size, bool unseekableStream)
         {
             // Write archive with a 8 Gb long entry.
diff --git a/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntryAsync.LongFile.Tests.cs b/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntryAsync.LongFile.Tests.cs
index 6d260cc9ab7bdc..837da5fea212fc 100644
--- a/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntryAsync.LongFile.Tests.cs
+++ b/src/libraries/System.Formats.Tar/tests/TarWriter/TarWriter.WriteEntryAsync.LongFile.Tests.cs
@@ -17,6 +17,7 @@ public static IEnumerable<object[]> WriteEntry_LongFileSize_TheoryDataAsync()
 
         [Theory]
         [MemberData(nameof(WriteEntry_LongFileSize_TheoryDataAsync))]
+        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.Android | TestPlatforms.Browser, "Needs too much disk space.")]
         public async Task WriteEntry_LongFileSizeAsync(TarEntryFormat entryFormat, long size, bool unseekableStream)
         {
             // Write archive with a 8 Gb long entry.
diff --git a/src/libraries/System.Management/tests/System/Management/ManagementEventWatcherTests.cs b/src/libraries/System.Management/tests/System/Management/ManagementEventWatcherTests.cs
index d91f3b5da038cc..ac2cfde2037885 100644
--- a/src/libraries/System.Management/tests/System/Management/ManagementEventWatcherTests.cs
+++ b/src/libraries/System.Management/tests/System/Management/ManagementEventWatcherTests.cs
@@ -12,6 +12,7 @@ public class ManagementEventWatcherTests
         private const string Query = "SELECT * FROM __TimerEvent WHERE TimerID = 'MyEvent'";
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsElevatedAndSupportsWmi))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Receive_Events_Sync()
         {
@@ -37,6 +38,7 @@ public void Receive_Events_Sync()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsElevatedAndSupportsWmi))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Receive_Events_Async()
         {
diff --git a/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTests.cs b/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTests.cs
index e119514aa1c89c..4bd99fb0c58465 100644
--- a/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTests.cs
+++ b/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTests.cs
@@ -25,6 +25,7 @@ public void Dynamic_Instances()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsWmiSupported))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Related_Instances()
         {
@@ -46,6 +47,7 @@ public void Related_Instances()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsWmiSupported))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Relationship_Classes()
         {
diff --git a/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTestsMofRequired.cs b/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTestsMofRequired.cs
index 747f6ff76a0bfa..bc01fda0da8c8d 100644
--- a/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTestsMofRequired.cs
+++ b/src/libraries/System.Management/tests/System/Management/ManagementObjectSearcherTestsMofRequired.cs
@@ -9,6 +9,7 @@ namespace System.Management.Tests
     public class ManagementObjectSearcherTestsMofRequired
     {
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsElevatedAndSupportsWmi))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Static_Instances()
         {
@@ -27,6 +28,7 @@ public void Static_Instances()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsElevatedAndSupportsWmi))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Static_Related_Instances()
         {
@@ -44,6 +46,7 @@ public void Static_Related_Instances()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsElevatedAndSupportsWmi))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Static_Relationship_Classes()
         {
diff --git a/src/libraries/System.Management/tests/System/Management/ManagementObjectTests.cs b/src/libraries/System.Management/tests/System/Management/ManagementObjectTests.cs
index 215ef2bbfe36ad..ccf8154335a87d 100644
--- a/src/libraries/System.Management/tests/System/Management/ManagementObjectTests.cs
+++ b/src/libraries/System.Management/tests/System/Management/ManagementObjectTests.cs
@@ -39,6 +39,7 @@ public void Get_Win32_LogicalDisk()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsWmiSupported))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void GetRelated_For_Win32_LogicalDisk()
         {
@@ -63,6 +64,7 @@ public void Set_Property_Win32_ComputerSystem()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsWmiSupported))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Invoke_Instance_And_Static_Method_Win32_Process()
         {
@@ -101,6 +103,7 @@ public void Invoke_Instance_And_Static_Method_Win32_Process()
         }
 
         [ConditionalFact(typeof(WmiTestHelper), nameof(WmiTestHelper.IsWmiSupported))]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/34689", TestPlatforms.Windows, TargetFrameworkMonikers.Netcoreapp, TestRuntimes.Mono)]
         [OuterLoop]
         public void Serialize_ManagementException()
         {
diff --git a/src/libraries/System.Memory/tests/EncodingExtensions/EncodingExtensionsTests.cs b/src/libraries/System.Memory/tests/EncodingExtensions/EncodingExtensionsTests.cs
index b3c8b9e33c9fcb..194adc1ead03fe 100644
--- a/src/libraries/System.Memory/tests/EncodingExtensions/EncodingExtensionsTests.cs
+++ b/src/libraries/System.Memory/tests/EncodingExtensions/EncodingExtensionsTests.cs
@@ -283,6 +283,7 @@ public static void GetBytes_Encoding_ReadOnlySequence_IBufferWriter_SingleSegmen
 
         [Fact]
         [OuterLoop] // this test takes ~10 seconds on modern hardware since it operates over GBs of data
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/79883", TestRuntimes.Mono)]
         public static void GetBytes_Encoding_ReadOnlySequence_IBufferWriter_LargeMultiSegment()
         {
             ReadOnlySequence<char> sequence = GetLargeRepeatingReadOnlySequence<char>(AllScalarsAsUtf16, 1500); // ~ 3.2bn chars of UTF-16 input
diff --git a/src/libraries/System.Net.Ping/tests/FunctionalTests/PingTest.cs b/src/libraries/System.Net.Ping/tests/FunctionalTests/PingTest.cs
index 43b4bb3be6d08a..49a95c858a9af8 100644
--- a/src/libraries/System.Net.Ping/tests/FunctionalTests/PingTest.cs
+++ b/src/libraries/System.Net.Ping/tests/FunctionalTests/PingTest.cs
@@ -741,7 +741,7 @@ public async Task SendPingToExternalHostWithLowTtlTest()
             options.Ttl = 1;
             // This should always fail unless host is one IP hop away.
             pingReply = await ping.SendPingAsync(host, TestSettings.PingTimeout, payload, options);
-            Assert.True(pingReply.Status == IPStatus.TimeExceeded || pingReply.Status == IPStatus.TtlExpired);
+            Assert.True(pingReply.Status == IPStatus.TimeExceeded || pingReply.Status == IPStatus.TtlExpired, $"pingReply.Status was {pingReply.Status} instead of TimeExceeded or TtlExpired");
             Assert.NotEqual(IPAddress.Any, pingReply.Address);
         }
 
diff --git a/src/libraries/System.Net.Sockets/tests/FunctionalTests/DualModeSocketTest.cs b/src/libraries/System.Net.Sockets/tests/FunctionalTests/DualModeSocketTest.cs
index 91ff1ce5d96e54..ccdc41c583892f 100644
--- a/src/libraries/System.Net.Sockets/tests/FunctionalTests/DualModeSocketTest.cs
+++ b/src/libraries/System.Net.Sockets/tests/FunctionalTests/DualModeSocketTest.cs
@@ -1235,7 +1235,7 @@ public void Socket_SendToAsyncV4IPEndPointToV4Host_Throws()
                 {
                     Assert.Equal(SocketError.Fault, args.SocketError);
                 }
-                else if (OperatingSystem.IsLinux())
+                else if (OperatingSystem.IsLinux() || OperatingSystem.IsAndroid())
                 {
                     // NOTE: on Linux, this API returns ENETUNREACH instead of EFAULT: this platform
                     //       checks the family of the provided socket address before checking its size
@@ -2085,7 +2085,7 @@ public void BeginReceiveMessageFromV6BoundToSpecificV4_NotReceived()
         }
 
         [Fact]
-        [SkipOnPlatform(TestPlatforms.Linux | TestPlatforms.OSX | TestPlatforms.MacCatalyst | TestPlatforms.iOS | TestPlatforms.tvOS, "Expected behavior is different on Apple platforms and Linux")]
+        [SkipOnPlatform(TestPlatforms.Linux | TestPlatforms.Android | TestPlatforms.OSX | TestPlatforms.MacCatalyst | TestPlatforms.iOS | TestPlatforms.tvOS, "Expected behavior is different on Apple platforms and Linux/Android")]
         public void BeginReceiveMessageFromV4BoundToSpecificV6_NotReceived()
         {
             Assert.Throws<TimeoutException>(() =>
@@ -2258,7 +2258,7 @@ public void ReceiveMessageFromAsyncV6BoundToSpecificV4_NotReceived()
         }
 
         [Fact]
-        [SkipOnPlatform(TestPlatforms.Linux | TestPlatforms.OSX | TestPlatforms.MacCatalyst | TestPlatforms.iOS | TestPlatforms.tvOS, "Expected behavior is different on Apple platforms and Linux")]
+        [SkipOnPlatform(TestPlatforms.Linux | TestPlatforms.Android | TestPlatforms.OSX | TestPlatforms.MacCatalyst | TestPlatforms.iOS | TestPlatforms.tvOS, "Expected behavior is different on Apple platforms and Linux/Android")]
         public void ReceiveMessageFromAsyncV4BoundToSpecificV6_NotReceived()
         {
             Assert.Throws<TimeoutException>(() =>
diff --git a/src/libraries/System.Runtime.Extensions/tests/System/EnvironmentTests.cs b/src/libraries/System.Runtime.Extensions/tests/System/EnvironmentTests.cs
index 55c666ba0b8b4a..97083e925d9e6c 100644
--- a/src/libraries/System.Runtime.Extensions/tests/System/EnvironmentTests.cs
+++ b/src/libraries/System.Runtime.Extensions/tests/System/EnvironmentTests.cs
@@ -368,6 +368,7 @@ public void GetFolderPath_Unix_PersonalIsDocumentsAndUserProfile()
         [InlineData(Environment.SpecialFolder.MyPictures)]
         [InlineData(Environment.SpecialFolder.MyVideos)]
         [InlineData(Environment.SpecialFolder.Templates)]
+        [SkipOnPlatform(TestPlatforms.iOS | TestPlatforms.tvOS | TestPlatforms.Android | TestPlatforms.Browser, "Not supported on iOS/tvOS/Android/Browser.")]
         public void GetFolderPath_Unix_SpecialFolderDoesNotExist_CreatesSuccessfully(Environment.SpecialFolder folder)
         {
             string path = Environment.GetFolderPath(folder, Environment.SpecialFolderOption.DoNotVerify);
diff --git a/src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs b/src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs
index 533d3c623df4d8..709b015a49dba7 100644
--- a/src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs
+++ b/src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs
@@ -500,6 +500,7 @@ public static void ToString_InvalidFormat_ThrowsFormatException()
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))] // Requires a lot of memory
         [OuterLoop("Takes a long time, allocates a lot of memory")]
+        [SkipOnMono("Frequently throws OOM on Mono")]
         public static void ToString_ValidLargeFormat()
         {
             BigInteger b = new BigInteger(123456789000m);
diff --git a/src/libraries/System.Runtime/tests/System/DoubleTests.cs b/src/libraries/System.Runtime/tests/System/DoubleTests.cs
index cff858f4e357d6..47b1a88366c26f 100644
--- a/src/libraries/System.Runtime/tests/System/DoubleTests.cs
+++ b/src/libraries/System.Runtime/tests/System/DoubleTests.cs
@@ -842,6 +842,7 @@ public static void ToString_InvalidFormat_ThrowsFormatException()
 
         [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))] // Requires a lot of memory
         [OuterLoop("Takes a long time, allocates a lot of memory")]
+        [SkipOnMono("Frequently throws OOM on Mono")]
         public static void ToString_ValidLargeFormat()
         {
             double d = 123.0;
diff --git a/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTests.cs b/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTests.cs
index 7b979c2a26b77a..bca63841dfde72 100644
--- a/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTests.cs
+++ b/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTests.cs
@@ -559,7 +559,7 @@ public void TestDecryptSimple_Pkcs7Signed_GeneratedOnLinux()
             VerifySimpleDecrypt(encodedMessage, certLoader, expectedContentInfo);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformSupport), nameof(PlatformSupport.IsRC2Supported))]
         [OuterLoop(/* Leaks key on disk if interrupted */)]
         public void TestDecryptSimpleAes256_RsaOaepSha256()
         {
@@ -580,7 +580,7 @@ public void TestDecryptSimpleAes256_RsaOaepSha256()
             VerifySimpleDecrypt(encodedMessage, Certificates.RSASha256KeyTransfer1, expectedContentInfo);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformSupport), nameof(PlatformSupport.IsRC2Supported))]
         [OuterLoop(/* Leaks key on disk if interrupted */)]
         public void TestDecryptSimpleAes256_RsaOaepSha384()
         {
@@ -603,7 +603,7 @@ public void TestDecryptSimpleAes256_RsaOaepSha384()
             VerifySimpleDecrypt(encodedMessage, Certificates.RSA2048Sha256KeyTransfer1, expectedContentInfo);
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformSupport), nameof(PlatformSupport.IsRC2Supported))]
         [OuterLoop(/* Leaks key on disk if interrupted */)]
         public void TestDecryptSimpleAes256_RsaOaepSha512()
         {
diff --git a/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTestsRsaPaddingModeTests.cs b/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTestsRsaPaddingModeTests.cs
index a19967c0441f27..0d6a5eaea71076 100644
--- a/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTestsRsaPaddingModeTests.cs
+++ b/src/libraries/System.Security.Cryptography.Pkcs/tests/EnvelopedCms/DecryptTestsRsaPaddingModeTests.cs
@@ -6,6 +6,8 @@
 using System.Security.Cryptography.X509Certificates;
 using Xunit;
 
+using Test.Cryptography;
+
 namespace System.Security.Cryptography.Pkcs.EnvelopedCmsTests.Tests
 {
     public class DecryptTestsRsaPaddingMode : DecryptTests
@@ -16,7 +18,7 @@ public DecryptTestsRsaPaddingMode() : base(false)
         {
         }
 
-        [Theory]
+        [ConditionalTheory(typeof(PlatformSupport), nameof(PlatformSupport.IsRC2Supported))]
         [MemberData(nameof(Roundtrip_RsaPaddingModes_TestData))]
         [OuterLoop(/* Leaks key on disk if interrupted */)]
         public static void Roundtrip_RsaPaddingModes(RSAEncryptionPadding rsaEncryptionPadding)
diff --git a/src/libraries/System.Security.Principal.Windows/tests/WindowsIdentityImpersonatedTests.netcoreapp.cs b/src/libraries/System.Security.Principal.Windows/tests/WindowsIdentityImpersonatedTests.netcoreapp.cs
index 5dd25c38787cd4..a1ecb9ae42c487 100644
--- a/src/libraries/System.Security.Principal.Windows/tests/WindowsIdentityImpersonatedTests.netcoreapp.cs
+++ b/src/libraries/System.Security.Principal.Windows/tests/WindowsIdentityImpersonatedTests.netcoreapp.cs
@@ -28,6 +28,7 @@ public WindowsIdentityImpersonatedTests(WindowsIdentityFixture windowsIdentityFi
 
     [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.CanRunImpersonatedTests))]
     [OuterLoop]
+    [ActiveIssue("https://github.com/dotnet/runtime/issues/80019", TestRuntimes.Mono)]
     public async Task RunImpersonatedAsync_TaskAndTaskOfT()
     {
         using WindowsIdentity currentWindowsIdentity = WindowsIdentity.GetCurrent();
@@ -69,6 +70,7 @@ void Asserts(WindowsIdentity currentWindowsIdentity)
 
     [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.CanRunImpersonatedTests))]
     [OuterLoop]
+    [ActiveIssue("https://github.com/dotnet/runtime/issues/80019", TestRuntimes.Mono)]
     public void RunImpersonated_NameResolution()
     {
         using WindowsIdentity currentWindowsIdentity = WindowsIdentity.GetCurrent();
@@ -90,6 +92,7 @@ public void RunImpersonated_NameResolution()
 
     [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.CanRunImpersonatedTests))]
     [OuterLoop]
+    [ActiveIssue("https://github.com/dotnet/runtime/issues/80019", TestRuntimes.Mono)]
     public async Task RunImpersonatedAsync_NameResolution()
     {
         using WindowsIdentity currentWindowsIdentity = WindowsIdentity.GetCurrent();
diff --git a/src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.AsyncEnumerable.cs b/src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.AsyncEnumerable.cs
index 0ae7c05046fd5d..91de0c3b21612b 100644
--- a/src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.AsyncEnumerable.cs
+++ b/src/libraries/System.Text.Json/tests/Common/CollectionTests/CollectionTests.AsyncEnumerable.cs
@@ -144,6 +144,7 @@ async IAsyncEnumerable<int> CreateEnumerable([EnumeratorCancellation] Cancellati
         [InlineData(5000, 1000, false)]
         [InlineData(1000, 10_000, true)]
         [InlineData(1000, 10_000, false)]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/80020", TestRuntimes.Mono)]
         public async Task WriteAsyncEnumerable_LongRunningEnumeration_Cancellation(
             int cancellationTokenSourceDelayMilliseconds,
             int enumeratorDelayMilliseconds,
diff --git a/src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.Cache.cs b/src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.Cache.cs
index 2f121926b4cc9b..5e111c289f5fef 100644
--- a/src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.Cache.cs
+++ b/src/libraries/System.Text.Json/tests/Common/ConstructorTests/ConstructorTests.Cache.cs
@@ -11,6 +11,7 @@ public abstract partial class ConstructorTests
     {
         [Fact]
         [OuterLoop]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/80020", TestRuntimes.Mono)]
         public async Task MultipleThreadsLooping()
         {
             const int Iterations = 100;
diff --git a/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.KnownPattern.Tests.cs b/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.KnownPattern.Tests.cs
index 16e333ad18db56..4cf860e0efc7f8 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.KnownPattern.Tests.cs
+++ b/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.KnownPattern.Tests.cs
@@ -1551,7 +1551,8 @@ public async Task PatternsDataSet_ConstructRegexForAll_NonBacktracking()
         }
 
         [OuterLoop("Takes minutes to generate and compile thousands of expressions")]
-        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))] // consumes a lot of memory
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess), nameof(PlatformDetection.IsNotMobile), nameof(PlatformDetection.IsNotBrowser))] // consumes a lot of memory, doesn't work on mobile
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/80018", TestRuntimes.Mono)]
         public void PatternsDataSet_ConstructRegexForAll_SourceGenerated()
         {
             Parallel.ForEach(s_patternsDataSet.Value.Chunk(50), chunk =>
diff --git a/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs b/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs
index cb928466b1054f..734b9d5e88d27b 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs
+++ b/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs
@@ -9,6 +9,7 @@
 using System.Threading.Tasks;
 using System.Tests;
 using Microsoft.DotNet.RemoteExecutor;
+using Microsoft.DotNet.XUnitExtensions;
 using Xunit;
 using Xunit.Sdk;
 
@@ -1097,7 +1098,7 @@ public async Task Match_VaryingLengthStrings(RegexEngine engine, RegexOptions op
 
         [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Takes several minutes on .NET Framework")]
         [OuterLoop("Takes several seconds")]
-        [Theory]
+        [ConditionalTheory]
         [MemberData(nameof(RegexHelpers.AvailableEngines_MemberData), MemberType = typeof(RegexHelpers))]
         public async Task Match_VaryingLengthStrings_Huge(RegexEngine engine)
         {
@@ -1125,6 +1126,11 @@ public async Task Match_VaryingLengthStrings_Huge(RegexEngine engine)
 
             if (RegexHelpers.IsNonBacktracking(engine))
             {
+                if (!RemoteExecutor.IsSupported)
+                {
+                    throw new SkipTestException("RemoteExecutor is not supported on this platform.");
+                }
+
                 RemoteExecutor.Invoke(func, engine.ToString()).Dispose();
             }
             else
@@ -2113,7 +2119,7 @@ public static IEnumerable<object[]> StressTestDeepNestingOfConcat_TestData()
         }
 
         [OuterLoop("Can take over a minute")]
-        [Theory]
+        [ConditionalTheory]
         [MemberData(nameof(StressTestDeepNestingOfConcat_TestData))]
         public async Task StressTestDeepNestingOfConcat(RegexEngine engine, string pattern, string anchor, string input, int pattern_repetition, int input_repetition)
         {
@@ -2147,6 +2153,11 @@ public async Task StressTestDeepNestingOfConcat(RegexEngine engine, string patte
 
             if (RegexHelpers.IsNonBacktracking(engine))
             {
+                if (!RemoteExecutor.IsSupported)
+                {
+                    throw new SkipTestException("RemoteExecutor is not supported on this platform.");
+                }
+
                 RemoteExecutor.Invoke(func, engine.ToString(), fullpattern, fullinput).Dispose();
             }
             else
@@ -2200,6 +2211,11 @@ public async Task StressTestDeepNestingOfLoops(RegexEngine engine, string begin,
 
             if (RegexHelpers.IsNonBacktracking(engine))
             {
+                if (!RemoteExecutor.IsSupported)
+                {
+                    throw new SkipTestException("RemoteExecutor is not supported on this platform.");
+                }
+
                 RemoteExecutor.Invoke(func, engine.ToString(), fullpattern, fullinput).Dispose();
             }
             else
diff --git a/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexIgnoreCaseTests.cs b/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexIgnoreCaseTests.cs
index 6262bb3df1af41..cf9a08339f7ff1 100644
--- a/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexIgnoreCaseTests.cs
+++ b/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexIgnoreCaseTests.cs
@@ -131,7 +131,7 @@ public async Task Ensure_CultureInvariant_Option_Is_Used_For_Backreferences(Rege
             Assert.True(regex.IsMatch("iI"));
         }
 
-        [Fact]
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsNotMobile), nameof(PlatformDetection.IsNotBrowser))]
         // This test creates a source generated engine for each of the ~870 cultures and ensures the result compiles. This test alone takes around 30
         // seconds on a fast machine, so marking as OuterLoop.
         [OuterLoop]
diff --git a/src/libraries/sendtohelixhelp.proj b/src/libraries/sendtohelixhelp.proj
index b2e072c486c843..7e7dcfedfb8b2e 100644
--- a/src/libraries/sendtohelixhelp.proj
+++ b/src/libraries/sendtohelixhelp.proj
@@ -39,7 +39,7 @@
     <_workItemTimeout Condition="'$(_workItemTimeout)' == '' and ('$(TargetOS)' == 'ios' or '$(TargetOS)' == 'tvos')">00:45:00</_workItemTimeout>
     <_workItemTimeout Condition="'$(Scenario)' == '' and '$(_workItemTimeout)' == '' and ('$(TargetArchitecture)' == 'arm64' or '$(TargetArchitecture)' == 'arm')">00:45:00</_workItemTimeout>
     <_workItemTimeout Condition="'$(Scenario)' != '' and '$(_workItemTimeout)' == '' and ('$(TargetArchitecture)' == 'arm64' or '$(TargetArchitecture)' == 'arm')">01:00:00</_workItemTimeout>
-    <_workItemTimeout Condition="'$(Scenario)' == '' and '$(_workItemTimeout)' == '' and '$(Outerloop)' == 'true'">00:20:00</_workItemTimeout>
+    <_workItemTimeout Condition="'$(Scenario)' == '' and '$(_workItemTimeout)' == '' and ('$(TestScope)' == 'outerloop' or '$(TestScope)' == 'all')">00:20:00</_workItemTimeout>
     <_workItemTimeout Condition="'$(Scenario)' == '' and '$(_workItemTimeout)' == ''">00:15:00</_workItemTimeout>
     <_workItemTimeout Condition="'$(Scenario)' != '' and '$(_workItemTimeout)' == ''">00:30:00</_workItemTimeout>
   </PropertyGroup>