diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..49ac8fc
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,27 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: astral-sh/ruff-action@v3
+
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.12"]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+      - run: pip install -e ".[test]"
+      - run: pytest tests/
diff --git a/.gitignore b/.gitignore
index 3ffa354..b36483e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .idea
 *.egg-info
 __pycache__
+build/
 tests/data/*renamed.jpg
diff --git a/README.md b/README.md
index 7a68f79..5b09978 100644
--- a/README.md
+++ b/README.md
@@ -125,6 +125,15 @@ im gray lena.jpg
 - Overwrite original file (be careful), using `-w`.
 - Use batch processing, using list of images (or globing), `im gray *.jpg`.
 
+## Development
+
+Run lint and tests locally:
+~~~bash
+pip install -e ".[test]"
+ruff check im/ tests/
+pytest tests/ -v
+~~~
+
 ## <a name="deps"></a>Dependencies
 All dependencies are standard pip installable packages. They are automatically installed with setup script.
 
diff --git a/im/im.py b/im/im.py
index 8906815..9da811f 100644
--- a/im/im.py
+++ b/im/im.py
@@ -1,13 +1,13 @@
+import argparse
+import multiprocessing as mp
 import os
-from functools import partial
 import shutil
 import sys
 import traceback
 from datetime import datetime
-import argparse
+from functools import partial
 
-from PIL import ImageOps, Image
-import multiprocessing as mp
+from PIL import Image, ImageOps
 
 from im.display import CursesDisplay
 from im.utils import *
@@ -51,6 +51,14 @@ def im_cmd():
     parser_exif.add_argument('--comment', '-c', help='Comment.', type=str, default=None)
     parser_exif.add_argument('--overwrite', '-w', help='Overwrite input images.', action='store_true')
 
+    parser_flip = subparsers.add_parser('flip', description='Flip image horizontally or vertically.',
+                                        help='Flip image horizontally or vertically.',
+                                        formatter_class=argparse.ArgumentDefaultsHelpFormatter)
+    parser_flip.set_defaults(func=flip)
+    parser_flip.add_argument('files', metavar='FILE', nargs='+', type=str)
+    parser_flip.add_argument('--vertical', '-v', help='Flip vertically (top to bottom).', action='store_true')
+    parser_flip.add_argument('--overwrite', '-w', help='Overwrite input images.', action='store_true')
+
     parser_rotate = subparsers.add_parser('rotate', description='Rotate image according to exif data',
                                           help='Rotate image according to exif data',
                                           formatter_class=argparse.ArgumentDefaultsHelpFormatter)
@@ -188,7 +196,7 @@ def stack(files: list, output: str, vertical: bool):
     sizes = [im.shape[i_shape] for im in ims]
     i_max = sizes.index(max(sizes))
     for i, im in enumerate(ims):
-        if i is not i_max:
+        if i != i_max:
             f = ims[i_max].shape[i_shape] / im.shape[i_shape]
             img = Image.fromarray(ims[i])
             h, w = list(ims[i].shape)[:2]
@@ -271,6 +279,22 @@ def exif(files: list, remove: bool, comment: str, overwrite: bool):
             _exif_show(exf)
 
 
+def flip(files: list, vertical: bool, overwrite: bool):
+    for m_input in files:
+        if overwrite:
+            out_file = m_input
+        else:
+            path_base, ext = os.path.splitext(m_input)
+            out_file = '%s_flipped%s' % (path_base, ext)
+        print(m_input, '-->', out_file, 'flipping ...')
+        image, exf = imread(m_input)
+        if vertical:
+            image = ImageOps.flip(image)
+        else:
+            image = ImageOps.mirror(image)
+        imwrite(image, out_file, exf)
+
+
 def rotate(files: list, overwrite: bool):
     for i, m_input in enumerate(files):
         if overwrite:
diff --git a/im/utils.py b/im/utils.py
index 7dbe2f2..838ca5d 100644
--- a/im/utils.py
+++ b/im/utils.py
@@ -1,6 +1,6 @@
 import numpy as np
-from PIL import Image
 import piexif
+from PIL import Image
 
 
 def imread(filepath):
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..cacd663
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,32 @@
+[tool.ruff]
+target-version = "py310"
+exclude = ["build"]
+
+[tool.ruff.lint]
+select = [
+    "E",    # pycodestyle errors
+    "F",    # pyflakes
+    "W",    # pycodestyle warnings
+    "I",    # isort
+    "UP",   # pyupgrade
+    "B",    # flake8-bugbear
+    "SIM",  # flake8-simplify
+    "PIE",  # flake8-pie
+]
+ignore = [
+    "E501",   # line too long (let formatter handle it)
+    "E722",   # bare except (used intentionally for image loading fallbacks)
+    "E741",   # ambiguous variable name (i, l, etc. are fine here)
+    "B007",   # unused loop variable
+    "F403",   # star imports (used intentionally for utils)
+    "F405",   # names from star imports (consequence of F403)
+    "SIM105", # use contextlib.suppress (not always clearer)
+    "SIM108", # ternary operator (not always clearer)
+    "UP031",  # printf string formatting (existing style)
+]
+
+[tool.ruff.lint.per-file-ignores]
+"tests/*" = ["E", "W"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
diff --git a/setup.py b/setup.py
index eff3609..7812069 100644
--- a/setup.py
+++ b/setup.py
@@ -14,6 +14,9 @@
         'Pillow>=11.1.0',
         'piexif>=1.1.3'
     ],
+    extras_require={
+        'test': ['pytest', 'ruff'],
+    },
     entry_points='''
         [console_scripts]
         im=im.im:im_cmd
diff --git a/tests/test_im.py b/tests/test_im.py
new file mode 100644
index 0000000..5432f44
--- /dev/null
+++ b/tests/test_im.py
@@ -0,0 +1,114 @@
+import os
+
+import numpy as np
+import pytest
+from PIL import Image
+
+from im.im import border, convert, crop, flip, gray, info, resize, stack
+
+
+@pytest.fixture
+def sample_image(tmp_path):
+    """Create a simple test RGB image."""
+    img = Image.fromarray(np.random.randint(0, 255, (100, 150, 3), dtype=np.uint8))
+    path = str(tmp_path / "test.png")
+    img.save(path)
+    return path
+
+
+@pytest.fixture
+def sample_image_jpg(tmp_path):
+    """Create a simple test RGB image in JPEG format."""
+    img = Image.fromarray(np.random.randint(0, 255, (100, 150, 3), dtype=np.uint8))
+    path = str(tmp_path / "test.jpg")
+    img.save(path)
+    return path
+
+
+def test_gray(sample_image, tmp_path):
+    gray(files=[sample_image], overwrite=False)
+    out = str(tmp_path / "test_gray.png")
+    assert os.path.exists(out)
+    img = Image.open(out)
+    assert img.mode == "L"
+
+
+def test_gray_overwrite(sample_image):
+    gray(files=[sample_image], overwrite=True)
+    img = Image.open(sample_image)
+    assert img.mode == "L"
+
+
+def test_resize(sample_image, tmp_path):
+    resize(files=[sample_image], overwrite=False, size=50, width=0, height=0)
+    out = str(tmp_path / "test_resized.png")
+    assert os.path.exists(out)
+    img = Image.open(out)
+    assert max(img.size) == 50
+
+
+def test_resize_explicit_dims(sample_image, tmp_path):
+    resize(files=[sample_image], overwrite=False, size=1000, width=80, height=60)
+    out = str(tmp_path / "test_resized.png")
+    img = Image.open(out)
+    assert img.size == (80, 60)
+
+
+def test_flip_horizontal(sample_image, tmp_path):
+    flip(files=[sample_image], vertical=False, overwrite=False)
+    out = str(tmp_path / "test_flipped.png")
+    assert os.path.exists(out)
+    original = np.asarray(Image.open(sample_image))
+    flipped = np.asarray(Image.open(out))
+    np.testing.assert_array_equal(flipped, original[:, ::-1, :])
+
+
+def test_flip_vertical(sample_image, tmp_path):
+    flip(files=[sample_image], vertical=True, overwrite=False)
+    out = str(tmp_path / "test_flipped.png")
+    original = np.asarray(Image.open(sample_image))
+    flipped = np.asarray(Image.open(out))
+    np.testing.assert_array_equal(flipped, original[::-1, :, :])
+
+
+def test_crop(sample_image, tmp_path):
+    crop(files=[sample_image], x=10, y=20, width=50, height=30, overwrite=False)
+    out = str(tmp_path / "test_cropped.png")
+    assert os.path.exists(out)
+    img = Image.open(out)
+    assert img.size == (50, 30)
+
+
+def test_border(sample_image, tmp_path):
+    border(files=[sample_image], width=5, color="red", overwrite=False)
+    out = str(tmp_path / "test_border.png")
+    assert os.path.exists(out)
+    img = Image.open(out)
+    assert img.size == (160, 110)  # 150+2*5, 100+2*5
+
+
+def test_convert(sample_image, tmp_path):
+    convert(files=[sample_image], extension=".jpg", overwrite=False)
+    out = str(tmp_path / "test.jpg")
+    assert os.path.exists(out)
+
+
+def test_stack_horizontal(tmp_path):
+    img1 = Image.fromarray(np.zeros((100, 50, 3), dtype=np.uint8))
+    img2 = Image.fromarray(np.zeros((100, 80, 3), dtype=np.uint8))
+    p1 = str(tmp_path / "a.png")
+    p2 = str(tmp_path / "b.png")
+    img1.save(p1)
+    img2.save(p2)
+    out = str(tmp_path / "stacked.png")
+    stack(files=[p1, p2], output=out, vertical=False)
+    assert os.path.exists(out)
+    result = Image.open(out)
+    assert result.size[1] == 100  # height preserved
+
+
+def test_info(sample_image, capsys):
+    info(files=[sample_image])
+    captured = capsys.readouterr()
+    assert "Size" in captured.out
+    assert "Mode" in captured.out