diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
new file mode 100644
index 0000000..349bede
--- /dev/null
+++ b/.github/workflows/test.yml
@@ -0,0 +1,28 @@
+name: Python package
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: ["3.9", "3.10", "3.11"]
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+      - name: Test with pytest
+        run: |
+          pytest tests/
diff --git a/.gitignore b/.gitignore
index 5249f05..1d267dc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -161,5 +161,5 @@ cython_debug/
 #  option (not recommended) you can uncomment the following to ignore the entire idea folder.
 #.idea/
 
-# demo, test folder
-!demo/**
\ No newline at end of file
+# testdata folder
+!tests/testdata/**
\ No newline at end of file
diff --git a/.vscode/settings.json b/.vscode/settings.json
new file mode 100644
index 0000000..d969f96
--- /dev/null
+++ b/.vscode/settings.json
@@ -0,0 +1,5 @@
+{
+  "python.testing.pytestArgs": ["tests"],
+  "python.testing.unittestEnabled": false,
+  "python.testing.pytestEnabled": true
+}
diff --git a/README.md b/README.md
index 6b20f11..3f32a89 100644
--- a/README.md
+++ b/README.md
@@ -78,6 +78,12 @@ pip install -r requirements.txt
 python -m files_flattener.cli <directory> <output_file> [<ignore_file>]
 ```
 
+#### Run the tests
+
+```sh
+pytest tests/
+```
+
 #### Build the package
 
 Ensure `wheel` is installed:
@@ -108,10 +114,9 @@ Upload the distribution files to PyPI:
 
 ## TODO
 
+- [ ] feat: Add support for spcifying the git repository to clone and flatten.
 - [ ] refactor: Use click package to implement the CLI.
-- [ ] Add dry-run mode to preview the output before writing to the file.
-
-- [x] Use `.ignore` to exclude or include files for flattening.
+- [ ] feat: Add dry-run mode to preview the output before writing to the file.
 
 ## License
 
diff --git a/demo/target/.ignore b/demo/target/.ignore
deleted file mode 100644
index 4045a45..0000000
--- a/demo/target/.ignore
+++ /dev/null
@@ -1,4 +0,0 @@
-folder1/file2.txt
-
-# Exclude itself
-.ignore
\ No newline at end of file
diff --git a/files_flattener/core.py b/files_flattener/core.py
index ef1dcd3..5f39266 100644
--- a/files_flattener/core.py
+++ b/files_flattener/core.py
@@ -2,7 +2,7 @@
 import pathspec
 from concurrent.futures import ThreadPoolExecutor
 
-from .directory_handler import (
+from .identifier_handler import (
     DirectoryHandlerFactory,
     LocalDirectoryHandler,
     RemoteRepositoryHandler,
diff --git a/files_flattener/directory_handler.py b/files_flattener/identifier_handler.py
similarity index 90%
rename from files_flattener/directory_handler.py
rename to files_flattener/identifier_handler.py
index 94ddf55..e07b359 100644
--- a/files_flattener/directory_handler.py
+++ b/files_flattener/identifier_handler.py
@@ -3,7 +3,7 @@
 from .common import logger
 
 
-class DirectoryHandler:
+class IdentifierHandler:
     def __init__(self, identifier):
         self.identifier = identifier
 
@@ -11,12 +11,12 @@ def prepare(self):
         raise NotImplementedError
 
 
-class LocalDirectoryHandler(DirectoryHandler):
+class LocalDirectoryHandler(IdentifierHandler):
     def prepare(self):
         self.local_directory = self.identifier
 
 
-class RemoteRepositoryHandler(DirectoryHandler):
+class RemoteRepositoryHandler(IdentifierHandler):
     def prepare(self):
         self.local_directory = tempfile.mkdtemp()
         logger.warning(
diff --git a/requirements.txt b/requirements.txt
index f4b36bd..c888c59 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,5 +3,6 @@ colorlog
 GitPython
 
 # For development
+pytest
 twine
 wheel
\ No newline at end of file
diff --git a/tests/absimport.py b/tests/absimport.py
new file mode 100644
index 0000000..5713f39
--- /dev/null
+++ b/tests/absimport.py
@@ -0,0 +1,4 @@
+import sys
+import os
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
diff --git a/tests/test_core.py b/tests/test_core.py
new file mode 100644
index 0000000..fdfa8d0
--- /dev/null
+++ b/tests/test_core.py
@@ -0,0 +1,18 @@
+import absimport  # FIRST LINE
+import pytest
+from files_flattener.core import list_files
+from files_flattener.identifier_handler import LocalDirectoryHandler
+
+
+@pytest.mark.parametrize(
+    "identifier, ignore_file, ignored",
+    [
+        ("tests/testdata/target", None, "folder1/file2.txt"),
+        ("tests/testdata/target", "tests/testdata/.ignore", "folder1/file2.txt"),
+    ],
+)
+def test_list_files(identifier, ignore_file, ignored):
+    files, handler = list_files(identifier, ignore_file)
+    assert isinstance(files, list)
+    assert isinstance(handler, LocalDirectoryHandler)
+    assert ignored not in files if ignore_file else ignored in files
diff --git a/tests/testdata/.ignore b/tests/testdata/.ignore
new file mode 100644
index 0000000..6179151
--- /dev/null
+++ b/tests/testdata/.ignore
@@ -0,0 +1 @@
+folder1/file2.txt
diff --git a/demo/output.txt b/tests/testdata/output.txt
similarity index 100%
rename from demo/output.txt
rename to tests/testdata/output.txt
diff --git a/demo/target/file3.txt b/tests/testdata/target/file3.txt
similarity index 100%
rename from demo/target/file3.txt
rename to tests/testdata/target/file3.txt
diff --git a/demo/target/folder1/file1.txt b/tests/testdata/target/folder1/file1.txt
similarity index 100%
rename from demo/target/folder1/file1.txt
rename to tests/testdata/target/folder1/file1.txt
diff --git a/demo/target/folder1/file2.txt b/tests/testdata/target/folder1/file2.txt
similarity index 100%
rename from demo/target/folder1/file2.txt
rename to tests/testdata/target/folder1/file2.txt