diff --git a/flatsurvey/limits.py b/flatsurvey/limits.py
index a2a51ec..e7bc7b7 100644
--- a/flatsurvey/limits.py
+++ b/flatsurvey/limits.py
@@ -1,17 +1,94 @@
+r"""
+Utilities to watch resource consumption on Linux machines.
+
+EXAMPLES:
+
+Limits can be used directly with the ``check`` method::
+
+    >>> from flatsurvey.limits import TimeLimit
+    >>> limit = TimeLimit(TimeLimit.parse_limit('1s'))
+    >>> limit.check()
+    True
+
+    >>> import time
+    >>> time.sleep(1)
+    >>> limit.check()
+    False
+
+Limits can also run in the background in async workflows::
+
+    >>> from flatsurvey.limits import TimeLimit, LimitChecker
+
+    >>> import asyncio
+
+    >>> async def main():
+    ...     LimitChecker(TimeLimit(TimeLimit.parse_limit('100ms')), lambda: print("callback executed"), period=1).start()
+    ...     print("working...")
+    ...     await asyncio.sleep(2)
+    ...     print("done.")
+
+    >>> asyncio.run(main())
+    working...
+    callback executed
+    done.
+
+"""
+# *********************************************************************
+#  This file is part of flatsurvey.
+#
+#        Copyright (C) 2024 Julian Rüth
+#
+#  flatsurvey is free software: you can redistribute it and/or modify
+#  it under the terms of the GNU General Public License as published by
+#  the Free Software Foundation, either version 3 of the License, or
+#  (at your option) any later version.
+#
+#  flatsurvey is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with flatsurvey. If not, see <https://www.gnu.org/licenses/>.
+# *********************************************************************
 import logging
 
 logger = logging.getLogger()
 
 
 class Limit:
+    r"""
+    Abstract base class for a limited resource that can be checked.
+    """
     def __init__(self, limit):
         self._limit = limit
 
     def check(self):
+        r"""
+        Return whether the limit has been reached.
+
+        EXAMPLES::
+
+            >>> from flatsurvey.limits import TimeLimit
+            >>> limit = TimeLimit(TimeLimit.parse_limit('1ms'))
+            >>> limit.check()
+            True
+
+            >>> import time
+            >>> time.sleep(.001)
+            >>> limit.check()
+            False
+
+        """
         raise NotImplementedError("this Limit does not implement check() yet")
 
 
 class LimitChecker:
+    r"""
+    Schedules a ``callback`` once the ``limit`` has been exceeded.
+
+    This can only be used in an ``async`` environment.
+    """
     def __init__(self, limit, callback, period=30):
         self._limit = limit
         self._callback = callback
@@ -20,6 +97,10 @@ def __init__(self, limit, callback, period=30):
         self._task = None
 
     def start(self):
+        r"""
+        Schedule the ``limit`` to be checked every ``period`` seconds in the
+        async event loop.
+        """
         assert self._task is None
 
         import asyncio
@@ -28,22 +109,23 @@ def start(self):
         self._task = loop.create_task(self._run())
 
     def stop(self):
+        r"""
+        Do not schedule this limit check any further.
+        """
         if self._task is not None:
             self._task.cancel()
             self._task = None
 
-    def check(self):
-        if not self._limit.check():
-            self._callback()
-            self.stop()
-
     async def _run(self):
         try:
             while True:
-                self.check()
+                assert self._task is not None
 
-                import asyncio
+                if not self._limit.check():
+                    self._callback()
+                    self.stop()
 
+                import asyncio
                 await asyncio.sleep(self._period)
         except Exception as e:
             import traceback
@@ -54,6 +136,9 @@ async def _run(self):
 
 
 class TimeLimit(Limit):
+    r"""
+    A wall time limit.
+    """
     def __init__(self, limit):
         super().__init__(limit)
 
@@ -61,11 +146,43 @@ def __init__(self, limit):
 
     @staticmethod
     def parse_limit(limit):
+        r"""
+        Helper method to parse ``limit`` into a Python time delta.
+
+        EXAMPLES::
+
+            >>> from flatsurvey.limits import TimeLimit
+            >>> TimeLimit.parse_limit('100ms')
+            datetime.timedelta(microseconds=100000)
+
+        """
         import pandas
 
         return pandas.Timedelta(limit).to_pytimedelta()
 
     def check(self):
+        r"""
+        Return whether the wall time has advanced by the configured limit since
+        this method was called for the first time.
+
+        EXAMPLES::
+
+            >>> from flatsurvey.limits import TimeLimit
+            >>> limit = TimeLimit(TimeLimit.parse_limit('10ms'))
+
+            >>> import time
+            >>> time.sleep(.01)
+
+            >>> limit.check()
+            True
+
+            >>> import time
+            >>> time.sleep(.01)
+
+            >>> limit.check()
+            False
+
+        """
         import time
 
         now = time.time()
@@ -82,6 +199,24 @@ def check(self):
 
 
 class MemoryLimit(Limit):
+    r"""
+    A limit on the total RAM and swap memory used by this process.
+
+    EXMAPLES::
+
+        >>> from flatsurvey.limits import MemoryLimit
+        >>> limit = MemoryLimit(2**28)
+        >>> limit.check()
+        True
+
+    Each int uses four bytes, so we exceed the memory limit by allocating that
+    many ints::
+
+        >>> A = [0] * 2**26
+        >>> limit.check()
+        False
+
+    """
     @staticmethod
     def parse_limit(limit):
         import psutil