diff --git a/ncc/include/string.h b/ncc/include/string.h
index bb54b96..f5706af 100644
--- a/ncc/include/string.h
+++ b/ncc/include/string.h
@@ -68,10 +68,34 @@ char* strchr(char *str, int c)
     return NULL;
 }
 
-// TODO:
 // Returns a pointer to the first occurrence of str2 in str1,
 // or a null pointer if str2 is not part of str1.
-// char* strstr(char * str1, const char * str2);
+char* strstr(char* s1, char* s2)
+{
+    char* p1 = s1;
+    char* p2;
+
+    while (*s1)
+    {
+        p2 = s2;
+
+        while (*p2 && (*p1 == *p2))
+        {
+            ++p1;
+            ++p2;
+        }
+
+        if (!*p2)
+        {
+            return (char*)s1;
+        }
+
+        ++s1;
+        p1 = s1;
+    }
+
+    return NULL;
+}
 
 // TODO:
 // char* strncpy(char* destination, const char* source, size_t num)
diff --git a/ncc/tests/strings.c b/ncc/tests/strings.c
index 1ce2e6e..b7bb550 100644
--- a/ncc/tests/strings.c
+++ b/ncc/tests/strings.c
@@ -28,10 +28,19 @@ int main()
 
     // strchr
     char* str = "lestring";
-    assert(strchr("", 'c') == null);
-    assert(strchr("c", 'c') != null);
+    assert(strchr("", 'c') == NULL);
+    assert(strchr("c", 'c') != NULL);
     assert(strchr(str, 's') == str + 2);
 
+    // strstr
+    char* s = "abcabcabcdabcde";
+    assert(strstr(s, "x") == NULL);
+    assert(strstr(s, "xyz") == NULL);
+    assert(strstr(s, "a") == s + 0);
+    assert(strstr(s, "abc") == s + 0);
+    assert(strstr(s, "abcd") == s + 6);
+    assert(strstr(s, "abcde") == s + 10);
+
     // memset
     memset(arr, 177, 19);
     assert(arr[0] == 177);