diff --git a/README.md b/README.md
index 5485bea..042b9d8 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,34 @@
 Files for tbaMUD.
 
+## Unit Tests
+
+tbaMUD ships with a C unit-test suite located in the `tests/` directory.
+The suite uses the [Unity](https://github.com/ThrowTheSwitch/Unity) test
+framework (vendored under `tests/vendor/unity/`).
+
+### Quick start
+
+```
+./configure
+cd tests && make test
+```
+
+`make test` builds each test binary, runs it, and writes JUnit XML results to
+`tests/test-results/`.  A summary is printed to the terminal:
+
+```
+[PASS] test_utils
+[PASS] test_random
+[PASS] test_interpreter
+[PASS] test_class
+```
+
+### CI
+
+The GitHub Actions workflow (`.github/workflows/build.yml`) runs `make test`
+on every push and pull request against `master` and publishes a formatted
+report via the `dorny/test-reporter` action.
+
+See [doc/testing.md](doc/testing.md) for full details on adding new tests and
+understanding the test infrastructure.
 
diff --git a/doc/testing.md b/doc/testing.md
new file mode 100644
index 0000000..de5452f
--- /dev/null
+++ b/doc/testing.md
@@ -0,0 +1,150 @@
+# tbaMUD Unit Testing
+
+_Updated 2026-04_
+
+## Overview
+
+tbaMUD has a C unit-test suite built on the
+[Unity](https://github.com/ThrowTheSwitch/Unity) framework.  Tests live in the
+`tests/` directory alongside the vendored Unity source.
+
+```
+tests/
+  Makefile.in          – Autoconf template; processed by configure
+  test_stubs.c         – Weak-symbol stubs that satisfy mud headers
+  unity_to_junit.py    – Converts Unity output to JUnit XML
+  test_class.c         – Tests for src/class.c
+  test_interpreter.c   – Tests for src/interpreter.c
+  test_random.c        – Tests for src/random.c
+  test_utils.c         – Tests for src/utils.c
+  vendor/unity/        – Vendored Unity test framework
+```
+
+## Prerequisites
+
+| Requirement | Notes |
+|---|---|
+| C compiler (gcc or clang) | Same compiler used to build the mud |
+| GNU make | Any POSIX-compatible make works |
+| Python 3 | Required only for JUnit XML conversion (`unity_to_junit.py`) |
+| autoconf / configure | Already needed to build the mud |
+
+## Running the tests
+
+Run `./configure` from the repository root first (only needed once):
+
+```sh
+./configure
+```
+
+Then build and run all tests from the `tests/` directory:
+
+```sh
+cd tests
+make test
+```
+
+`make test` performs the following steps for each test binary:
+
+1. Compiles the test binary (if not already up to date).
+2. Runs the binary and captures stdout/stderr to `test-results/<name>.out`.
+3. Measures wall-clock elapsed time.
+4. Converts the Unity output to JUnit XML via `unity_to_junit.py`, writing
+   `test-results/<name>.xml`.
+5. Prints `[PASS] <name>` or `[FAIL] <name>` and exits non-zero if any
+   binary failed.
+
+To build the test binaries without running them:
+
+```sh
+cd tests
+make
+```
+
+To remove all test binaries and result files:
+
+```sh
+cd tests
+make clean
+```
+
+## Test suites
+
+| Binary | Source under test | Test file |
+|---|---|---|
+| `test_utils` | `src/utils.c`, `src/random.c` | `test_utils.c` |
+| `test_random` | `src/random.c`, `rand_number`/`dice` in `src/utils.c` | `test_random.c` |
+| `test_interpreter` | `src/interpreter.c` | `test_interpreter.c` |
+| `test_class` | `src/class.c` | `test_class.c` |
+
+## Writing a new test
+
+### Adding a test case to an existing suite
+
+1. Open the relevant `test_<name>.c` file.
+2. Write a function with the signature `void test_my_feature(void)`.
+3. Use Unity assertion macros such as `TEST_ASSERT_EQUAL_INT`,
+   `TEST_ASSERT_NULL`, `TEST_ASSERT_TRUE`, etc.
+4. Register the function in the `main()` block:
+   ```c
+   RUN_TEST(test_my_feature);
+   ```
+
+Example:
+
+```c
+void test_str_cmp_equal_strings(void)
+{
+    TEST_ASSERT_EQUAL_INT(0, str_cmp("hello", "hello"));
+}
+```
+
+### Creating a new test suite
+
+1. Create `tests/test_<module>.c`.  Copy the boilerplate from an existing
+   suite: include `unity.h`, define `setUp`/`tearDown` (may be empty), write
+   test functions, and provide a `main()` that calls `UNITY_BEGIN()`,
+   `RUN_TEST(...)` for each function, and `return UNITY_END();`.
+
+2. Add the binary to `tests/Makefile.in`:
+   - Add the name to the `TESTS` variable.
+   - Add a build rule:
+     ```make
+     test_<module>: $(UNITY_SRC) $(STUBS_SRC) $(UTILS_SRC) \
+                    $(SRCDIR)/<module>.c test_<module>.c
+         $(COMPILE) -o $@ $^ $(LIBS)
+     ```
+
+3. Re-run `./configure` from the repository root to regenerate
+   `tests/Makefile` from the updated `tests/Makefile.in`.
+
+### Stubs
+
+Many mud source files reference global variables and functions that are only
+meaningful at runtime (e.g. `descriptor_list`, `log()`).  `test_stubs.c`
+provides zero-initialised definitions and `__attribute__((weak))` stub
+implementations for these symbols so that test binaries link without pulling
+in the full mud.
+
+If a new test requires a function not yet stubbed, add a weak stub to
+`test_stubs.c`:
+
+```c
+__attribute__((weak)) void my_function(void) { /* no-op */ }
+```
+
+## JUnit XML output and CI
+
+`unity_to_junit.py` reads Unity's line-oriented output on stdin and writes a
+JUnit-compatible XML file.  It accepts an optional third argument with the
+elapsed wall-clock time in seconds (provided by the `make test` target):
+
+```
+usage: unity_to_junit.py <suite_name> <output.xml> [elapsed_seconds]
+```
+
+The GitHub Actions workflow (`.github/workflows/build.yml`) runs `make test`
+on every push and pull request against `master`.  After the tests finish the
+`dorny/test-reporter` action reads `tests/test-results/*.xml` and publishes a
+formatted report as a GitHub Check with pass/fail counts and per-suite
+execution times.