--- /dev/null
+Tests
+=====
+
+## Directory structure
+
+- [`test/unit`](./unit): Unit tests
+ - [`test/unit/test.vimspec`](./unit/test.vimspec): Unit test cases
+ - [`test/unit/runtime`](./unit/runtime): Runtime directory loaded on running unit tests. They mocks several external APIs like `ale#*` or `lsp#*`
+- [`test/integ`](./integ): Integration tests
+ - [`test/integ/test.vimspec`](./integ/test.vimspec): Integration test cases
+ - [`test/integ/deps`](./integ/deps): Dependant plugins
+
+## Unit tests
+
+Unit tests confirm vim-lsp-ale works as intended.
+
+### Prerequisites
+
+Unit tests can be run with no dependency because they mock every external API.
+
+[vim-themis](https://github.com/thinca/vim-themis) is used as test runner.
+
+By default, it runs tests with `vim` command. When running tests with Neovim, set `THEMIS_VIM=nvim` environment variable.
+
+### Run unit tests
+
+```sh
+cd path/to/vim-lsp-ale
+git clone https://github.com/thinca/vim-themis.git
+
+# Run tests with Vim
+./vim-themis/bin/themis ./test/unit/
+
+# Run tests with NeoVim
+THEMIS_VIM=nvim ./vim-themis/bin/themis ./test/unit/
+```
+
+### Measure unit test coverage
+
+[covimerage](https://github.com/Vimjas/covimerage) is used to measure test coverage. Install it by `pip install covimerage`.
+
+Set a file path to `THEMIS_PROFILE` environment variable and run unit tests. Vim will store the profile data to the file.
+`covimerage` command will convert the profile data into coverage data for `coverage` command provided by Python.
+
+```sh
+cd path/to/vim-lsp-ale
+git clone https://github.com/thinca/vim-themis.git
+
+# Run test case with $THEMIS_PROFILE environment variable
+THEMIS_PROFILE=profile.txt ./vim-themis/bin/themis ./test/unit
+
+# Store coverage data at .coverage_covimerage converted from the profile data
+covimerage write_coverage profile.txt
+
+# Show coverage report by `coverage` command
+coverage report
+```
+
+## Integration tests
+
+Integration tests confirm integrity among vim-lsp, ALE, vim-lsp-ale and a language server.
+
+### Prerequisites
+
+Integration tests require all dependencies are installed in [deps](./integ/deps) directory.
+
+```sh
+cd path/to/vim-lsp-ale
+git clone https://github.com/prabirshrestha/vim-lsp.git test/integ/deps/vim-lsp
+git clone https://github.com/dense-analysis/ale.git test/integ/deps/ale
+```
+
+[rust-analyzer](https://rust-analyzer.github.io/) is used as language server to run integration tests.
+Download the binary following [the instruction](https://rust-analyzer.github.io/manual.html#rust-analyzer-language-server-binary)
+and put the binary in `$PATH` directory.
+
+And [vim-themis](https://github.com/thinca/vim-themis) is used as test runner.
+
+Note that integration tests were not confirmed on Windows.
+
+### Run integration tests
+
+```sh
+cd path/to/vim-lsp-ale
+git clone https://github.com/thinca/vim-themis.git
+./vim-themis/bin/themis ./test/integ/
+```
+
+### Log files
+
+When some integration tests fail, the following log files would be useful to investigate the failure.
+
+- `test/integ/integ_messages.txt`: Messages in `:message` area while running the tests
+- `test/integ/lsp-log.txt`: Log information of vim-lsp. It records communications between vim-lsp and a language server
+
+## CI
+
+Tests are run continuously on GitHub Actions.
+
+https://github.com/rhysd/vim-lsp-ale/actions?query=workflow%3ACI
+
+- Unit tests are run on Linux, macOS and Windows with Vim and Neovim
+- Integration tests are run on Linux with Vim and Neovim