diff --git a/.coveragerc b/.coveragerc
index de12642263..19c173c00d 100644
--- a/.coveragerc
+++ b/.coveragerc
@@ -1,6 +1,6 @@
 [run]
 branch=True
-source=trio
+source=src/trio
 omit=
   setup.py
 # These are run in subprocesses, but still don't work. We follow
diff --git a/docs/source/conf.py b/docs/source/conf.py
index 66aa8dea05..82ec6a23a7 100755
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -22,7 +22,7 @@
 # For our local_customization module
 sys.path.insert(0, os.path.abspath("."))
 # For trio itself
-sys.path.insert(0, os.path.abspath("../.."))
+sys.path.insert(0, os.path.abspath("../../src"))
 
 # https://docs.readthedocs.io/en/stable/builds.html#build-environment
 if "READTHEDOCS" in os.environ:
diff --git a/pyproject.toml b/pyproject.toml
index 79beab840d..3edc1329a7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -13,13 +13,13 @@ ignore-words-list = 'astroid,crasher,asend'
 [tool.flake8]
 extend-ignore = ['D', 'E', 'W', 'F403', 'F405', 'F821', 'F822']
 per-file-ignores = [
-  'trio/__init__.py: F401',
-  'trio/_core/__init__.py: F401',
-  'trio/_core/_tests/test_multierror_scripts/*: F401',
-  'trio/abc.py: F401',
-  'trio/lowlevel.py: F401',
-  'trio/socket.py: F401',
-  'trio/testing/__init__.py: F401'
+  'src/trio/__init__.py: F401',
+  'src/trio/_core/__init__.py: F401',
+  'src/trio/_core/_tests/test_multierror_scripts/*: F401',
+  'src/trio/abc.py: F401',
+  'src/trio/lowlevel.py: F401',
+  'src/trio/socket.py: F401',
+  'src/trio/testing/__init__.py: F401'
 ]
 
 [tool.isort]
@@ -61,48 +61,48 @@ disallow_untyped_calls = false
 [[tool.mypy.overrides]]
 module = [
 # 2761
-"trio/_core/_generated_io_windows",
-"trio/_core/_io_windows",
+"src/trio/_core/_generated_io_windows",
+"src/trio/_core/_io_windows",
 
 # internal
-"trio/_windows_pipes",
+"src/trio/_windows_pipes",
 
 # tests
-"trio/testing/_fake_net",
-"trio/_core/_tests/test_guest_mode",
-"trio/_core/_tests/test_instrumentation",
-"trio/_core/_tests/test_ki",
-"trio/_core/_tests/test_local",
-"trio/_core/_tests/test_mock_clock",
-"trio/_core/_tests/test_multierror",
-"trio/_core/_tests/test_multierror_scripts/ipython_custom_exc",
-"trio/_core/_tests/test_multierror_scripts/simple_excepthook",
-"trio/_core/_tests/test_parking_lot",
-"trio/_core/_tests/test_thread_cache",
-"trio/_core/_tests/test_unbounded_queue",
-"trio/_tests/test_exports",
-"trio/_tests/test_file_io",
-"trio/_tests/test_highlevel_generic",
-"trio/_tests/test_highlevel_open_tcp_listeners",
-"trio/_tests/test_highlevel_open_tcp_stream",
-"trio/_tests/test_highlevel_open_unix_stream",
-"trio/_tests/test_highlevel_serve_listeners",
-"trio/_tests/test_highlevel_socket",
-"trio/_tests/test_highlevel_ssl_helpers",
-"trio/_tests/test_path",
-"trio/_tests/test_scheduler_determinism",
-"trio/_tests/test_socket",
-"trio/_tests/test_ssl",
-"trio/_tests/test_subprocess",
-"trio/_tests/test_sync",
-"trio/_tests/test_testing",
-"trio/_tests/test_threads",
-"trio/_tests/test_timeouts",
-"trio/_tests/test_tracing",
-"trio/_tests/test_util",
-"trio/_tests/test_wait_for_object",
-"trio/_tests/test_windows_pipes",
-"trio/_tests/tools/test_gen_exports",
+"src/trio/testing/_fake_net",
+"src/trio/_core/_tests/test_guest_mode",
+"src/trio/_core/_tests/test_instrumentation",
+"src/trio/_core/_tests/test_ki",
+"src/trio/_core/_tests/test_local",
+"src/trio/_core/_tests/test_mock_clock",
+"src/trio/_core/_tests/test_multierror",
+"src/trio/_core/_tests/test_multierror_scripts/ipython_custom_exc",
+"src/trio/_core/_tests/test_multierror_scripts/simple_excepthook",
+"src/trio/_core/_tests/test_parking_lot",
+"src/trio/_core/_tests/test_thread_cache",
+"src/trio/_core/_tests/test_unbounded_queue",
+"src/trio/_tests/test_exports",
+"src/trio/_tests/test_file_io",
+"src/trio/_tests/test_highlevel_generic",
+"src/trio/_tests/test_highlevel_open_tcp_listeners",
+"src/trio/_tests/test_highlevel_open_tcp_stream",
+"src/trio/_tests/test_highlevel_open_unix_stream",
+"src/trio/_tests/test_highlevel_serve_listeners",
+"src/trio/_tests/test_highlevel_socket",
+"src/trio/_tests/test_highlevel_ssl_helpers",
+"src/trio/_tests/test_path",
+"src/trio/_tests/test_scheduler_determinism",
+"src/trio/_tests/test_socket",
+"src/trio/_tests/test_ssl",
+"src/trio/_tests/test_subprocess",
+"src/trio/_tests/test_sync",
+"src/trio/_tests/test_testing",
+"src/trio/_tests/test_threads",
+"src/trio/_tests/test_timeouts",
+"src/trio/_tests/test_tracing",
+"src/trio/_tests/test_util",
+"src/trio/_tests/test_wait_for_object",
+"src/trio/_tests/test_windows_pipes",
+"src/trio/_tests/tools/test_gen_exports",
 ]
 check_untyped_defs = false
 disallow_any_decorated = false
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
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