diff --git a/setup.py b/setup.py
index dbce61c0fd..f6cb207375 100644
--- a/setup.py
+++ b/setup.py
@@ -1,6 +1,6 @@
 from setuptools import find_packages, setup
 
-exec(open("trio/_version.py", encoding="utf-8").read())
+exec(open("src/trio/_version.py", encoding="utf-8").read())
 
 LONG_DESC = """\
 .. image:: https://raw.githubusercontent.com/python-trio/trio/9b0bec646a31e0d0f67b8b6ecc6939726faf3e17/logo/logo-with-background.svg
@@ -80,7 +80,8 @@
     author_email="njs@pobox.com",
     url="https://github.com/python-trio/trio",
     license="MIT OR Apache-2.0",
-    packages=find_packages(),
+    packages=find_packages(where="src"),
+    package_dir={"": "src"},
     install_requires=[
         # attrs 19.2.0 adds `eq` option to decorators
         # attrs 20.1.0 adds @frozen
diff --git a/trio/__init__.py b/src/trio/__init__.py
similarity index 100%
rename from trio/__init__.py
rename to src/trio/__init__.py
diff --git a/trio/_abc.py b/src/trio/_abc.py
similarity index 100%
rename from trio/_abc.py
rename to src/trio/_abc.py
diff --git a/trio/_channel.py b/src/trio/_channel.py
similarity index 100%
rename from trio/_channel.py
rename to src/trio/_channel.py
diff --git a/trio/_core/__init__.py b/src/trio/_core/__init__.py
similarity index 100%
rename from trio/_core/__init__.py
rename to src/trio/_core/__init__.py
diff --git a/trio/_core/_asyncgens.py b/src/trio/_core/_asyncgens.py
similarity index 100%
rename from trio/_core/_asyncgens.py
rename to src/trio/_core/_asyncgens.py
diff --git a/trio/_core/_entry_queue.py b/src/trio/_core/_entry_queue.py
similarity index 100%
rename from trio/_core/_entry_queue.py
rename to src/trio/_core/_entry_queue.py
diff --git a/trio/_core/_exceptions.py b/src/trio/_core/_exceptions.py
similarity index 100%
rename from trio/_core/_exceptions.py
rename to src/trio/_core/_exceptions.py
diff --git a/trio/_core/_generated_instrumentation.py b/src/trio/_core/_generated_instrumentation.py
similarity index 100%
rename from trio/_core/_generated_instrumentation.py
rename to src/trio/_core/_generated_instrumentation.py
diff --git a/trio/_core/_generated_io_epoll.py b/src/trio/_core/_generated_io_epoll.py
similarity index 100%
rename from trio/_core/_generated_io_epoll.py
rename to src/trio/_core/_generated_io_epoll.py
diff --git a/trio/_core/_generated_io_kqueue.py b/src/trio/_core/_generated_io_kqueue.py
similarity index 100%
rename from trio/_core/_generated_io_kqueue.py
rename to src/trio/_core/_generated_io_kqueue.py
diff --git a/trio/_core/_generated_io_windows.py b/src/trio/_core/_generated_io_windows.py
similarity index 100%
rename from trio/_core/_generated_io_windows.py
rename to src/trio/_core/_generated_io_windows.py
diff --git a/trio/_core/_generated_run.py b/src/trio/_core/_generated_run.py
similarity index 100%
rename from trio/_core/_generated_run.py
rename to src/trio/_core/_generated_run.py
diff --git a/trio/_core/_instrumentation.py b/src/trio/_core/_instrumentation.py
similarity index 100%
rename from trio/_core/_instrumentation.py
rename to src/trio/_core/_instrumentation.py
diff --git a/trio/_core/_io_common.py b/src/trio/_core/_io_common.py
similarity index 100%
rename from trio/_core/_io_common.py
rename to src/trio/_core/_io_common.py
diff --git a/trio/_core/_io_epoll.py b/src/trio/_core/_io_epoll.py
similarity index 100%
rename from trio/_core/_io_epoll.py
rename to src/trio/_core/_io_epoll.py
diff --git a/trio/_core/_io_kqueue.py b/src/trio/_core/_io_kqueue.py
similarity index 100%
rename from trio/_core/_io_kqueue.py
rename to src/trio/_core/_io_kqueue.py
diff --git a/trio/_core/_io_windows.py b/src/trio/_core/_io_windows.py
similarity index 100%
rename from trio/_core/_io_windows.py
rename to src/trio/_core/_io_windows.py
diff --git a/trio/_core/_ki.py b/src/trio/_core/_ki.py
similarity index 100%
rename from trio/_core/_ki.py
rename to src/trio/_core/_ki.py
diff --git a/trio/_core/_local.py b/src/trio/_core/_local.py
similarity index 100%
rename from trio/_core/_local.py
rename to src/trio/_core/_local.py
diff --git a/trio/_core/_mock_clock.py b/src/trio/_core/_mock_clock.py
similarity index 100%
rename from trio/_core/_mock_clock.py
rename to src/trio/_core/_mock_clock.py
diff --git a/trio/_core/_multierror.py b/src/trio/_core/_multierror.py
similarity index 100%
rename from trio/_core/_multierror.py
rename to src/trio/_core/_multierror.py
diff --git a/trio/_core/_parking_lot.py b/src/trio/_core/_parking_lot.py
similarity index 100%
rename from trio/_core/_parking_lot.py
rename to src/trio/_core/_parking_lot.py
diff --git a/trio/_core/_run.py b/src/trio/_core/_run.py
similarity index 100%
rename from trio/_core/_run.py
rename to src/trio/_core/_run.py
diff --git a/trio/_core/_tests/__init__.py b/src/trio/_core/_tests/__init__.py
similarity index 100%
rename from trio/_core/_tests/__init__.py
rename to src/trio/_core/_tests/__init__.py
diff --git a/trio/_core/_tests/test_asyncgen.py b/src/trio/_core/_tests/test_asyncgen.py
similarity index 100%
rename from trio/_core/_tests/test_asyncgen.py
rename to src/trio/_core/_tests/test_asyncgen.py
diff --git a/trio/_core/_tests/test_guest_mode.py b/src/trio/_core/_tests/test_guest_mode.py
similarity index 100%
rename from trio/_core/_tests/test_guest_mode.py
rename to src/trio/_core/_tests/test_guest_mode.py
diff --git a/trio/_core/_tests/test_instrumentation.py b/src/trio/_core/_tests/test_instrumentation.py
similarity index 100%
rename from trio/_core/_tests/test_instrumentation.py
rename to src/trio/_core/_tests/test_instrumentation.py
diff --git a/trio/_core/_tests/test_io.py b/src/trio/_core/_tests/test_io.py
similarity index 100%
rename from trio/_core/_tests/test_io.py
rename to src/trio/_core/_tests/test_io.py
diff --git a/trio/_core/_tests/test_ki.py b/src/trio/_core/_tests/test_ki.py
similarity index 100%
rename from trio/_core/_tests/test_ki.py
rename to src/trio/_core/_tests/test_ki.py
diff --git a/trio/_core/_tests/test_local.py b/src/trio/_core/_tests/test_local.py
similarity index 100%
rename from trio/_core/_tests/test_local.py
rename to src/trio/_core/_tests/test_local.py
diff --git a/trio/_core/_tests/test_mock_clock.py b/src/trio/_core/_tests/test_mock_clock.py
similarity index 100%
rename from trio/_core/_tests/test_mock_clock.py
rename to src/trio/_core/_tests/test_mock_clock.py
diff --git a/trio/_core/_tests/test_multierror.py b/src/trio/_core/_tests/test_multierror.py
similarity index 100%
rename from trio/_core/_tests/test_multierror.py
rename to src/trio/_core/_tests/test_multierror.py
diff --git a/trio/_core/_tests/test_multierror_scripts/__init__.py b/src/trio/_core/_tests/test_multierror_scripts/__init__.py
similarity index 100%
rename from trio/_core/_tests/test_multierror_scripts/__init__.py
rename to src/trio/_core/_tests/test_multierror_scripts/__init__.py
diff --git a/trio/_core/_tests/test_multierror_scripts/_common.py b/src/trio/_core/_tests/test_multierror_scripts/_common.py
similarity index 100%
rename from trio/_core/_tests/test_multierror_scripts/_common.py
rename to src/trio/_core/_tests/test_multierror_scripts/_common.py
diff --git a/trio/_core/_tests/test_multierror_scripts/apport_excepthook.py b/src/trio/_core/_tests/test_multierror_scripts/apport_excepthook.py
similarity index 100%
rename from trio/_core/_tests/test_multierror_scripts/apport_excepthook.py
rename to src/trio/_core/_tests/test_multierror_scripts/apport_excepthook.py
diff --git a/trio/_core/_tests/test_multierror_scripts/simple_excepthook.py b/src/trio/_core/_tests/test_multierror_scripts/simple_excepthook.py
similarity index 100%
rename from trio/_core/_tests/test_multierror_scripts/simple_excepthook.py
rename to src/trio/_core/_tests/test_multierror_scripts/simple_excepthook.py
diff --git a/trio/_core/_tests/test_parking_lot.py b/src/trio/_core/_tests/test_parking_lot.py
similarity index 100%
rename from trio/_core/_tests/test_parking_lot.py
rename to src/trio/_core/_tests/test_parking_lot.py
diff --git a/trio/_core/_tests/test_run.py b/src/trio/_core/_tests/test_run.py
similarity index 100%
rename from trio/_core/_tests/test_run.py
rename to src/trio/_core/_tests/test_run.py
diff --git a/trio/_core/_tests/test_thread_cache.py b/src/trio/_core/_tests/test_thread_cache.py
similarity index 100%
rename from trio/_core/_tests/test_thread_cache.py
rename to src/trio/_core/_tests/test_thread_cache.py
diff --git a/trio/_core/_tests/test_tutil.py b/src/trio/_core/_tests/test_tutil.py
similarity index 100%
rename from trio/_core/_tests/test_tutil.py
rename to src/trio/_core/_tests/test_tutil.py
diff --git a/trio/_core/_tests/test_unbounded_queue.py b/src/trio/_core/_tests/test_unbounded_queue.py
similarity index 100%
rename from trio/_core/_tests/test_unbounded_queue.py
rename to src/trio/_core/_tests/test_unbounded_queue.py
diff --git a/trio/_core/_tests/test_windows.py b/src/trio/_core/_tests/test_windows.py
similarity index 100%
rename from trio/_core/_tests/test_windows.py
rename to src/trio/_core/_tests/test_windows.py
diff --git a/trio/_core/_tests/tutil.py b/src/trio/_core/_tests/tutil.py
similarity index 100%
rename from trio/_core/_tests/tutil.py
rename to src/trio/_core/_tests/tutil.py
diff --git a/trio/_core/_thread_cache.py b/src/trio/_core/_thread_cache.py
similarity index 100%
rename from trio/_core/_thread_cache.py
rename to src/trio/_core/_thread_cache.py
diff --git a/trio/_core/_traps.py b/src/trio/_core/_traps.py
similarity index 100%
rename from trio/_core/_traps.py
rename to src/trio/_core/_traps.py
diff --git a/trio/_core/_unbounded_queue.py b/src/trio/_core/_unbounded_queue.py
similarity index 100%
rename from trio/_core/_unbounded_queue.py
rename to src/trio/_core/_unbounded_queue.py
diff --git a/trio/_core/_wakeup_socketpair.py b/src/trio/_core/_wakeup_socketpair.py
similarity index 100%
rename from trio/_core/_wakeup_socketpair.py
rename to src/trio/_core/_wakeup_socketpair.py
diff --git a/trio/_core/_windows_cffi.py b/src/trio/_core/_windows_cffi.py
similarity index 100%
rename from trio/_core/_windows_cffi.py
rename to src/trio/_core/_windows_cffi.py
diff --git a/trio/_deprecate.py b/src/trio/_deprecate.py
similarity index 100%
rename from trio/_deprecate.py
rename to src/trio/_deprecate.py
diff --git a/trio/_dtls.py b/src/trio/_dtls.py
similarity index 100%
rename from trio/_dtls.py
rename to src/trio/_dtls.py
diff --git a/trio/_file_io.py b/src/trio/_file_io.py
similarity index 100%
rename from trio/_file_io.py
rename to src/trio/_file_io.py
diff --git a/trio/_highlevel_generic.py b/src/trio/_highlevel_generic.py
similarity index 100%
rename from trio/_highlevel_generic.py
rename to src/trio/_highlevel_generic.py
diff --git a/trio/_highlevel_open_tcp_listeners.py b/src/trio/_highlevel_open_tcp_listeners.py
similarity index 100%
rename from trio/_highlevel_open_tcp_listeners.py
rename to src/trio/_highlevel_open_tcp_listeners.py
diff --git a/trio/_highlevel_open_tcp_stream.py b/src/trio/_highlevel_open_tcp_stream.py
similarity index 100%
rename from trio/_highlevel_open_tcp_stream.py
rename to src/trio/_highlevel_open_tcp_stream.py
diff --git a/trio/_highlevel_open_unix_stream.py b/src/trio/_highlevel_open_unix_stream.py
similarity index 100%
rename from trio/_highlevel_open_unix_stream.py
rename to src/trio/_highlevel_open_unix_stream.py
diff --git a/trio/_highlevel_serve_listeners.py b/src/trio/_highlevel_serve_listeners.py
similarity index 100%
rename from trio/_highlevel_serve_listeners.py
rename to src/trio/_highlevel_serve_listeners.py
diff --git a/trio/_highlevel_socket.py b/src/trio/_highlevel_socket.py
similarity index 100%
rename from trio/_highlevel_socket.py
rename to src/trio/_highlevel_socket.py
diff --git a/trio/_highlevel_ssl_helpers.py b/src/trio/_highlevel_ssl_helpers.py
similarity index 100%
rename from trio/_highlevel_ssl_helpers.py
rename to src/trio/_highlevel_ssl_helpers.py
diff --git a/trio/_path.py b/src/trio/_path.py
similarity index 100%
rename from trio/_path.py
rename to src/trio/_path.py
diff --git a/trio/_signals.py b/src/trio/_signals.py
similarity index 100%
rename from trio/_signals.py
rename to src/trio/_signals.py
diff --git a/trio/_socket.py b/src/trio/_socket.py
similarity index 100%
rename from trio/_socket.py
rename to src/trio/_socket.py
diff --git a/trio/_ssl.py b/src/trio/_ssl.py
similarity index 100%
rename from trio/_ssl.py
rename to src/trio/_ssl.py
diff --git a/trio/_subprocess.py b/src/trio/_subprocess.py
similarity index 100%
rename from trio/_subprocess.py
rename to src/trio/_subprocess.py
diff --git a/trio/_subprocess_platform/__init__.py b/src/trio/_subprocess_platform/__init__.py
similarity index 100%
rename from trio/_subprocess_platform/__init__.py
rename to src/trio/_subprocess_platform/__init__.py
diff --git a/trio/_subprocess_platform/kqueue.py b/src/trio/_subprocess_platform/kqueue.py
similarity index 100%
rename from trio/_subprocess_platform/kqueue.py
rename to src/trio/_subprocess_platform/kqueue.py
diff --git a/trio/_subprocess_platform/waitid.py b/src/trio/_subprocess_platform/waitid.py
similarity index 100%
rename from trio/_subprocess_platform/waitid.py
rename to src/trio/_subprocess_platform/waitid.py
diff --git a/trio/_subprocess_platform/windows.py b/src/trio/_subprocess_platform/windows.py
similarity index 100%
rename from trio/_subprocess_platform/windows.py
rename to src/trio/_subprocess_platform/windows.py
diff --git a/trio/_sync.py b/src/trio/_sync.py
similarity index 100%
rename from trio/_sync.py
rename to src/trio/_sync.py
diff --git a/trio/_tests/__init__.py b/src/trio/_tests/__init__.py
similarity index 100%
rename from trio/_tests/__init__.py
rename to src/trio/_tests/__init__.py
diff --git a/trio/_tests/astrill-codesigning-cert.cer b/src/trio/_tests/astrill-codesigning-cert.cer
similarity index 100%
rename from trio/_tests/astrill-codesigning-cert.cer
rename to src/trio/_tests/astrill-codesigning-cert.cer
diff --git a/trio/_tests/check_type_completeness.py b/src/trio/_tests/check_type_completeness.py
similarity index 100%
rename from trio/_tests/check_type_completeness.py
rename to src/trio/_tests/check_type_completeness.py
diff --git a/trio/_tests/module_with_deprecations.py b/src/trio/_tests/module_with_deprecations.py
similarity index 100%
rename from trio/_tests/module_with_deprecations.py
rename to src/trio/_tests/module_with_deprecations.py
diff --git a/trio/_tests/pytest_plugin.py b/src/trio/_tests/pytest_plugin.py
similarity index 100%
rename from trio/_tests/pytest_plugin.py
rename to src/trio/_tests/pytest_plugin.py
diff --git a/trio/_tests/test_abc.py b/src/trio/_tests/test_abc.py
similarity index 100%
rename from trio/_tests/test_abc.py
rename to src/trio/_tests/test_abc.py
diff --git a/trio/_tests/test_channel.py b/src/trio/_tests/test_channel.py
similarity index 100%
rename from trio/_tests/test_channel.py
rename to src/trio/_tests/test_channel.py
diff --git a/trio/_tests/test_contextvars.py b/src/trio/_tests/test_contextvars.py
similarity index 100%
rename from trio/_tests/test_contextvars.py
rename to src/trio/_tests/test_contextvars.py
diff --git a/trio/_tests/test_deprecate.py b/src/trio/_tests/test_deprecate.py
similarity index 100%
rename from trio/_tests/test_deprecate.py
rename to src/trio/_tests/test_deprecate.py
diff --git a/trio/_tests/test_dtls.py b/src/trio/_tests/test_dtls.py
similarity index 100%
rename from trio/_tests/test_dtls.py
rename to src/trio/_tests/test_dtls.py
diff --git a/trio/_tests/test_exports.py b/src/trio/_tests/test_exports.py
similarity index 100%
rename from trio/_tests/test_exports.py
rename to src/trio/_tests/test_exports.py
diff --git a/trio/_tests/test_fakenet.py b/src/trio/_tests/test_fakenet.py
similarity index 100%
rename from trio/_tests/test_fakenet.py
rename to src/trio/_tests/test_fakenet.py
diff --git a/trio/_tests/test_file_io.py b/src/trio/_tests/test_file_io.py
similarity index 100%
rename from trio/_tests/test_file_io.py
rename to src/trio/_tests/test_file_io.py
diff --git a/trio/_tests/test_highlevel_generic.py b/src/trio/_tests/test_highlevel_generic.py
similarity index 100%
rename from trio/_tests/test_highlevel_generic.py
rename to src/trio/_tests/test_highlevel_generic.py
diff --git a/trio/_tests/test_highlevel_open_tcp_listeners.py b/src/trio/_tests/test_highlevel_open_tcp_listeners.py
similarity index 100%
rename from trio/_tests/test_highlevel_open_tcp_listeners.py
rename to src/trio/_tests/test_highlevel_open_tcp_listeners.py
diff --git a/trio/_tests/test_highlevel_open_tcp_stream.py b/src/trio/_tests/test_highlevel_open_tcp_stream.py
similarity index 100%
rename from trio/_tests/test_highlevel_open_tcp_stream.py
rename to src/trio/_tests/test_highlevel_open_tcp_stream.py
diff --git a/trio/_tests/test_highlevel_open_unix_stream.py b/src/trio/_tests/test_highlevel_open_unix_stream.py
similarity index 100%
rename from trio/_tests/test_highlevel_open_unix_stream.py
rename to src/trio/_tests/test_highlevel_open_unix_stream.py
diff --git a/trio/_tests/test_highlevel_serve_listeners.py b/src/trio/_tests/test_highlevel_serve_listeners.py
similarity index 100%
rename from trio/_tests/test_highlevel_serve_listeners.py
rename to src/trio/_tests/test_highlevel_serve_listeners.py
diff --git a/trio/_tests/test_highlevel_socket.py b/src/trio/_tests/test_highlevel_socket.py
similarity index 100%
rename from trio/_tests/test_highlevel_socket.py
rename to src/trio/_tests/test_highlevel_socket.py
diff --git a/trio/_tests/test_highlevel_ssl_helpers.py b/src/trio/_tests/test_highlevel_ssl_helpers.py
similarity index 100%
rename from trio/_tests/test_highlevel_ssl_helpers.py
rename to src/trio/_tests/test_highlevel_ssl_helpers.py
diff --git a/trio/_tests/test_path.py b/src/trio/_tests/test_path.py
similarity index 100%
rename from trio/_tests/test_path.py
rename to src/trio/_tests/test_path.py
diff --git a/trio/_tests/test_scheduler_determinism.py b/src/trio/_tests/test_scheduler_determinism.py
similarity index 100%
rename from trio/_tests/test_scheduler_determinism.py
rename to src/trio/_tests/test_scheduler_determinism.py
diff --git a/trio/_tests/test_signals.py b/src/trio/_tests/test_signals.py
similarity index 100%
rename from trio/_tests/test_signals.py
rename to src/trio/_tests/test_signals.py
diff --git a/trio/_tests/test_socket.py b/src/trio/_tests/test_socket.py
similarity index 100%
rename from trio/_tests/test_socket.py
rename to src/trio/_tests/test_socket.py
diff --git a/trio/_tests/test_ssl.py b/src/trio/_tests/test_ssl.py
similarity index 100%
rename from trio/_tests/test_ssl.py
rename to src/trio/_tests/test_ssl.py
diff --git a/trio/_tests/test_subprocess.py b/src/trio/_tests/test_subprocess.py
similarity index 100%
rename from trio/_tests/test_subprocess.py
rename to src/trio/_tests/test_subprocess.py
diff --git a/trio/_tests/test_sync.py b/src/trio/_tests/test_sync.py
similarity index 100%
rename from trio/_tests/test_sync.py
rename to src/trio/_tests/test_sync.py
diff --git a/trio/_tests/test_testing.py b/src/trio/_tests/test_testing.py
similarity index 100%
rename from trio/_tests/test_testing.py
rename to src/trio/_tests/test_testing.py
diff --git a/trio/_tests/test_threads.py b/src/trio/_tests/test_threads.py
similarity index 100%
rename from trio/_tests/test_threads.py
rename to src/trio/_tests/test_threads.py
diff --git a/trio/_tests/test_timeouts.py b/src/trio/_tests/test_timeouts.py
similarity index 100%
rename from trio/_tests/test_timeouts.py
rename to src/trio/_tests/test_timeouts.py
diff --git a/trio/_tests/test_tracing.py b/src/trio/_tests/test_tracing.py
similarity index 100%
rename from trio/_tests/test_tracing.py
rename to src/trio/_tests/test_tracing.py
diff --git a/trio/_tests/test_unix_pipes.py b/src/trio/_tests/test_unix_pipes.py
similarity index 100%
rename from trio/_tests/test_unix_pipes.py
rename to src/trio/_tests/test_unix_pipes.py
diff --git a/trio/_tests/test_util.py b/src/trio/_tests/test_util.py
similarity index 100%
rename from trio/_tests/test_util.py
rename to src/trio/_tests/test_util.py
diff --git a/trio/_tests/test_wait_for_object.py b/src/trio/_tests/test_wait_for_object.py
similarity index 100%
rename from trio/_tests/test_wait_for_object.py
rename to src/trio/_tests/test_wait_for_object.py
diff --git a/trio/_tests/test_windows_pipes.py b/src/trio/_tests/test_windows_pipes.py
similarity index 100%
rename from trio/_tests/test_windows_pipes.py
rename to src/trio/_tests/test_windows_pipes.py
diff --git a/trio/_tests/tools/__init__.py b/src/trio/_tests/tools/__init__.py
similarity index 100%
rename from trio/_tests/tools/__init__.py
rename to src/trio/_tests/tools/__init__.py
diff --git a/trio/_tests/tools/test_gen_exports.py b/src/trio/_tests/tools/test_gen_exports.py
similarity index 100%
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diff --git a/trio/_version.py b/src/trio/_version.py
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diff --git a/trio/from_thread.py b/src/trio/from_thread.py
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