diff --git a/README.md b/README.md
index 450fb1ce..4e9d85a7 100644
--- a/README.md
+++ b/README.md
@@ -20,6 +20,7 @@
 * [Can be enabled or disabled on production.][runtime]
 * [Colorless](colorless): annotate only what you want. Hence, easy integration into an existing project.
 * Colorful: syntax highlighting for every piece of code in every command.
+* [Memory leaks detection.][leaks] Deal makes sure that a pure function doesn't leave unexpected objects in the memory.
 * DRY: test discovery, error messages generation.
 * Partial execution: linter executes contracts to statically check possible values.
 
@@ -31,6 +32,7 @@
 [module_load]: https://deal.readthedocs.io/details/module_load.html
 [runtime]: https://deal.readthedocs.io/basic/runtime.html
 [colorless]: http://journal.stuffwithstuff.com/2015/02/01/what-color-is-your-function/
+[leaks]: https://deal.readthedocs.io/basic/tests.html#memory-leaks
 
 ## Deal in 30 seconds
 
diff --git a/deal/_cli/_main.py b/deal/_cli/_main.py
index f9201a12..acc69612 100644
--- a/deal/_cli/_main.py
+++ b/deal/_cli/_main.py
@@ -7,11 +7,13 @@
 from ._lint import lint_command
 from ._stub import stub_command
 from ._test import test_command
+from ._memtest import memtest_command
 
 
 CommandsType = Mapping[str, Callable[[Sequence[str]], int]]
 COMMANDS: CommandsType = MappingProxyType(dict(
     lint=lint_command,
+    memtest=memtest_command,
     stub=stub_command,
     test=test_command,
 ))
diff --git a/deal/_cli/_memtest.py b/deal/_cli/_memtest.py
new file mode 100644
index 00000000..5be5d1a2
--- /dev/null
+++ b/deal/_cli/_memtest.py
@@ -0,0 +1,118 @@
+# built-in
+from contextlib import suppress
+import sys
+from argparse import ArgumentParser
+from importlib import import_module
+from pathlib import Path
+from typing import Dict, Iterator, Sequence, TextIO
+
+# app
+from .._testing import cases, TestCase
+from .._mem_test import MemoryTracker
+from .._state import state
+from ..linter._extractors.pre import format_call_args
+from ._common import get_paths
+from .._colors import COLORS
+from ._test import sys_path, get_func_names
+
+
+def run_tests(path: Path, root: Path, count: int, stream: TextIO = sys.stdout) -> int:
+    names = list(get_func_names(path=path))
+    if not names:
+        return 0
+    print('{magenta}running {path}{end}'.format(path=path, **COLORS), file=stream)
+    module_name = '.'.join(path.relative_to(root).with_suffix('').parts)
+    with sys_path(path=root):
+        module = import_module(module_name)
+    failed = 0
+    for func_name in names:
+        func = getattr(module, func_name)
+        ok = run_cases(
+            cases=cases(func=func, count=count, check_types=False),
+            func_name=func_name,
+            stream=stream,
+            colors=COLORS,
+        )
+        if not ok:
+            failed += 1
+    return failed
+
+
+def run_cases(
+    cases: Iterator[TestCase],
+    func_name: str,
+    stream: TextIO,
+    colors: Dict[str, str],
+) -> bool:
+    print('  {blue}running {name}{end}'.format(name=func_name, **colors), file=stream)
+    for case in cases:
+        tracker = MemoryTracker()
+        debug = state.debug
+        state.disable()
+        try:
+            with tracker, suppress(Exception):
+                case()
+        finally:
+            state.debug = debug
+        if not tracker.diff:
+            continue
+
+        # show the diff and stop testing the func
+        line = '    {yellow}{name}({args}){end}'.format(
+            name=func_name,
+            args=format_call_args(args=case.args, kwargs=case.kwargs),
+            **colors,
+        )
+        print(line, file=stream)
+        longest_name_len = max(len(name) for name in tracker.diff)
+        for name, count in tracker.diff.items():
+            line = '      {red}{name}{end} x{count}'.format(
+                name=name.ljust(longest_name_len),
+                count=count,
+                **colors,
+            )
+            print(line, file=stream)
+        return False
+    return True
+
+
+def memtest_command(
+    argv: Sequence[str], root: Path = None, stream: TextIO = sys.stdout,
+) -> int:
+    """Generate and run tests against pure functions and report memory leaks.
+
+    ```bash
+    python3 -m deal memtest project/
+    ```
+
+    Function must be decorated by one of the following to be run:
+
+    + `@deal.pure`
+    + `@deal.has()` (without arguments)
+
+    Options:
+
+    + `--count`: how many input values combinations should be checked.
+
+    Exit code is equal to count of leaked functions.
+    See [memory leaks][leaks] documentation for more details.
+
+    [leaks]: https://deal.readthedocs.io/details/tests.html#memory-leaks
+    """
+    if root is None:  # pragma: no cover
+        root = Path()
+    parser = ArgumentParser(prog='python3 -m deal test')
+    parser.add_argument('--count', type=int, default=50)
+    parser.add_argument('paths', nargs='+')
+    args = parser.parse_args(argv)
+
+    failed = 0
+    for arg in args.paths:
+        for path in get_paths(Path(arg)):
+            failed += run_tests(
+                path=Path(path),
+                root=root,
+                count=args.count,
+                stream=stream,
+            )
+    return failed
diff --git a/deal/_exceptions.py b/deal/_exceptions.py
index 20ca0c52..7d4db3ec 100644
--- a/deal/_exceptions.py
+++ b/deal/_exceptions.py
@@ -28,7 +28,7 @@ def exception_hook(etype: Type[BaseException], value: BaseException, tb):
         if path.startswith(root):
             with suppress(AttributeError):  # read-only attribute in <3.7
                 prev_tb.tb_next = None
-            break
+            break  # pragma: no cover
         prev_tb = patched_tb
         patched_tb = patched_tb.tb_next
     else:
diff --git a/deal/_mem_test.py b/deal/_mem_test.py
new file mode 100644
index 00000000..3e3c4b80
--- /dev/null
+++ b/deal/_mem_test.py
@@ -0,0 +1,32 @@
+import gc
+import typing
+from collections import Counter
+from._cached_property import cached_property
+
+
+class MemoryTracker:
+    before: typing.Counter[str]
+    after: typing.Counter[str]
+
+    def __init__(self) -> None:
+        self.before = Counter()
+        self.after = Counter()
+
+    def __enter__(self) -> None:
+        self.before = self._dump()
+
+    def __exit__(self, *exc) -> None:
+        self.after = self._dump()
+
+    @cached_property
+    def diff(self) -> typing.Counter[str]:
+        return self.after - self.before - Counter({'weakref': 1})
+
+    @classmethod
+    def _dump(cls) -> typing.Counter[str]:
+        counter: typing.Counter[str] = Counter()
+        gc.collect()
+        for obj in gc.get_objects():
+            name: str = type(obj).__qualname__
+            counter[name] += 1
+        return counter
diff --git a/docs/basic/intro.md b/docs/basic/intro.md
index d6f21076..a87bdbe0 100644
--- a/docs/basic/intro.md
+++ b/docs/basic/intro.md
@@ -41,6 +41,7 @@ It's not "advanced usage", there is nothing advanced or difficult. It's about wr
 
 1. [module_load](../details/module_load) allows you to control what happens at the module load (import) time.
 1. [Stubs](../details/stubs) is a way to store some contracts in a JSON file instead of the source code. It can be helpful for third-party libraries. Some stubs already inside Deal.
+1. [More about testing](../details/tests) provides information on finding memory leaks and tweaking tests generation.
 1. [Validators](../details/validators) is a way to describe complex contracts using [Marshmallow](https://github.com/marshmallow-code/marshmallow) or another validation library.
 1. [Recipes](../details/recipes) is the place to learn more about best practices of using contracts.
 
diff --git a/docs/details/cli.md b/docs/details/cli.md
index 4529aafc..c0ad7cdf 100644
--- a/docs/details/cli.md
+++ b/docs/details/cli.md
@@ -17,3 +17,9 @@
 ```eval_rst
 .. autofunction:: deal._cli._test.test_command
 ```
+
+## memtest
+
+```eval_rst
+.. autofunction:: deal._cli._memtest.memtest_command
+```
diff --git a/docs/details/tests.md b/docs/details/tests.md
new file mode 100644
index 00000000..c967d7d9
--- /dev/null
+++ b/docs/details/tests.md
@@ -0,0 +1,34 @@
+# More about testing
+
+This section assumes that you're familiar with [basic testing](../basic/tests.md) and describes how you can get more from deal testing mechanisms.
+
+## Finding memory leaks
+
+Sometimes, when a function is completed, it leaves in memory other objects except result. For example:
+
+```python
+cache = {}
+User = dict
+
+def get_user(name: str) -> User:
+    if name not in cache:
+        cache[name] = User(name=name)
+    return cache[name]
+```
+
+Here, `get_user` creates a `User` object and stores it in a global cache. In this case, this "leak" is a desired behavior and we don't want to fight it. This is why we can't a tool (or something right in the Python interpreter) that catches and reports such behavior, it would have too many false-positives.
+
+However, things are different with pure functions. A pure function can't store anything on a side because it is a side effect. The result of a pure function is only what it returns.
+
+The command `memtest` uses this idea to find memory leaks in pure functions. How it works:
+
+1. It finds all pure functions (as `test` does).
+1. For every function:
+    1. It makes memory snapshot before running the function.
+    1. It runs the function with different autogenerated input arguments (as `test` command does) without running contracts and checking the return value type (to avoid side-effects from deal itself).
+    1. It makes memory snapshot after running the function.
+    1. Snapshots "before" and "after" are comapared. If there is a difference it will be printed.
+
+The return code is equal to the amount of functions with memory leaks.
+
+If the function fails, the command will ignore it and still test the function for leaks. Side-effects shouldn't happen unconditionally, even if the function fails. If you want to find unexpected failures, use `test` command instead.
diff --git a/docs/index.md b/docs/index.md
index 75aff6d1..3dbf93a3 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -33,6 +33,7 @@
     details/module_load
     details/stubs
     details/validators
+    details/tests
     details/recipes
     details/examples
     details/cli
diff --git a/tests/test_cli/test_mem_test.py b/tests/test_cli/test_mem_test.py
new file mode 100644
index 00000000..0949870e
--- /dev/null
+++ b/tests/test_cli/test_mem_test.py
@@ -0,0 +1,80 @@
+import sys
+from pathlib import Path
+from deal._cli._memtest import memtest_command
+from textwrap import dedent
+from io import StringIO
+
+
+def test_has_side_effect(tmp_path: Path, capsys):
+    if 'example' in sys.modules:
+        del sys.modules['example']
+    text = """
+        import deal
+
+        a = []
+
+        @deal.pure
+        def func(b: int) -> float:
+            a.append({b, b+b})
+            return None
+    """
+    path = (tmp_path / 'example.py')
+    path.write_text(dedent(text))
+    stream = StringIO()
+    result = memtest_command(['--count', '1', str(path)], root=tmp_path, stream=stream)
+    assert result == 1
+
+    stream.seek(0)
+    captured = stream.read()
+    assert '/example.py' in captured
+    assert 'running func' in captured
+    assert 'func(b=0)' in captured
+    assert 'set' in captured
+    assert 'x1' in captured
+
+
+def test_no_side_effects(tmp_path: Path, capsys):
+    if 'example' in sys.modules:
+        del sys.modules['example']
+    text = """
+        import deal
+
+        @deal.pure
+        def func(b: int) -> float:
+            return b+b
+    """
+    path = (tmp_path / 'example.py')
+    path.write_text(dedent(text))
+    stream = StringIO()
+    result = memtest_command(['--count', '1', str(path)], root=tmp_path, stream=stream)
+    assert result == 0
+
+    stream.seek(0)
+    captured = stream.read()
+    assert '/example.py' in captured
+    assert 'running func' in captured
+    assert 'func(b=0)' not in captured
+
+
+def test_no_matching_funcs(tmp_path: Path):
+    if 'example' in sys.modules:
+        del sys.modules['example']
+    text = """
+        import deal
+
+        def not_pure1(a: int, b: int) -> float:
+            return a / b
+
+        @deal.post(lambda result: result > 0)
+        def not_pure2(a: int, b: int) -> float:
+            return a / b
+    """
+    path = (tmp_path / 'example.py')
+    path.write_text(dedent(text))
+    stream = StringIO()
+    result = memtest_command([str(path)], root=tmp_path, stream=stream)
+    assert result == 0
+
+    stream.seek(0)
+    captured = stream.read()
+    assert '/example.py' not in captured
diff --git a/tests/test_linter/test_contract.py b/tests/test_linter/test_contract.py
index dacf271b..8e117246 100644
--- a/tests/test_linter/test_contract.py
+++ b/tests/test_linter/test_contract.py
@@ -224,8 +224,8 @@ def f(a):
     assert len(func.contracts) == 1
     c = func.contracts[0]
 
-    c.run(12) is False
-    c.run(34) is True
+    assert c.run(12) is False
+    assert c.run(34) is True
 
 
 def test_resolve_and_run_dependencies_lambda():
@@ -251,8 +251,8 @@ def f(a):
         assert len(func.contracts) == 1
         c = func.contracts[0]
 
-        c.run(12) is False
-        c.run(34) is True
+        assert c.run(12) is False
+        assert c.run(34) is True
 
 
 def test_lazy_import_stdlib():
@@ -270,8 +270,8 @@ def f(a):
     assert len(func.contracts) == 1
     c = func.contracts[0]
 
-    c.run('bcd') is False
-    c.run('abc') is True
+    assert c.run('bcd') is False
+    assert c.run('abc') is True
 
 
 def test_unresolvable():
diff --git a/tests/test_mem_test.py b/tests/test_mem_test.py
new file mode 100644
index 00000000..d5c0cb95
--- /dev/null
+++ b/tests/test_mem_test.py
@@ -0,0 +1,36 @@
+from deal._mem_test import MemoryTracker
+
+
+def test_mem_dump_no_diff():
+    def f():
+        return 123
+
+    tracker = MemoryTracker()
+    with tracker:
+        f()
+    assert not tracker.diff
+
+
+def test_mem_dump_ignore_locals():
+    def f():
+        a = 456
+        b = a
+        return b
+
+    tracker = MemoryTracker()
+    with tracker:
+        f()
+    assert not tracker.diff
+
+
+def test_mem_dump_side_effect():
+    a = []
+
+    def f():
+        a.append({12})
+        return 123
+
+    tracker = MemoryTracker()
+    with tracker:
+        f()
+    assert dict(tracker.diff) == {'set': 1}