diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..52c4941
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,47 @@
+name: CI
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - python-version: "3.8"
+            os: ubuntu-latest
+          - python-version: "3.9"
+            os: ubuntu-latest
+          - python-version: "3.10"
+            os: ubuntu-latest
+          - python-version: "3.11"
+            os: ubuntu-latest
+          - python-version: "3.12"
+            os: ubuntu-latest
+          - python-version: "3.13"
+            os: ubuntu-latest
+          - python-version: "3.14"
+            os: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+          allow-prereleases: true
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+          pip install pytest pytest-cov
+
+      - name: Run tests with coverage
+        run: pytest tests/ --cov=matterhook --cov-report=term-missing
diff --git a/README.rst b/README.rst
index 62a7c40..548073a 100644
--- a/README.rst
+++ b/README.rst
@@ -71,4 +71,25 @@ Advanced usage
     '''
     message['text'] = markdown_msg
     attachments.append(message)
-    mwh.send(attachments=attachments)
\ No newline at end of file
+    mwh.send(attachments=attachments)
+
+Testing
+=======
+
+Install the test dependencies first:
+
+.. code-block:: bash
+
+    pip install pytest pytest-cov
+
+Run the tests:
+
+.. code-block:: bash
+
+    pytest tests/
+
+Run the tests with code coverage:
+
+.. code-block:: bash
+
+    pytest tests/ --cov=matterhook --cov-report=term-missing
\ No newline at end of file
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..fed528d
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
diff --git a/run_tests.sh b/run_tests.sh
new file mode 100755
index 0000000..743abdf
--- /dev/null
+++ b/run_tests.sh
@@ -0,0 +1,25 @@
+#!/bin/bash
+
+set -e
+
+VERSIONS="3.8 3.9 3.10 3.11 3.12 3.13 3.14"
+FAILED=""
+
+for v in $VERSIONS; do
+    echo "=== Python $v ==="
+    if uv run --python "$v" --with pytest,pytest-cov,requests \
+        pytest tests/ --cov=matterhook --cov-report=term-missing; then
+        echo "=== Python $v: OK ==="
+    else
+        echo "=== Python $v: FAILED ==="
+        FAILED="$FAILED $v"
+    fi
+    echo
+done
+
+if [ -n "$FAILED" ]; then
+    echo "FAILED versions:$FAILED"
+    exit 1
+else
+    echo "All versions passed."
+fi
diff --git a/setup.py b/setup.py
index 776524c..0890e67 100644
--- a/setup.py
+++ b/setup.py
@@ -3,11 +3,8 @@
 """
 
 import os
-import sys
 
-from pkg_resources import get_distribution, parse_version
 from setuptools import find_packages, setup
-from setuptools.command.test import test as TestCommand
 
 
 def read(fname: str):
@@ -25,29 +22,6 @@ def read(fname: str):
     return open(os.path.join(os.path.dirname(__file__), fname)).read()
 
 
-class PyTest(TestCommand):
-    user_options = [("pytest-args=", "a", "Arguments to pass to py.test")]
-
-    def initialize_options(self):
-        TestCommand.initialize_options(self)
-        self.pytest_args = []
-
-    def finalize_options(self):
-        TestCommand.finalize_options(self)
-
-        # https://bitbucket.org/pypa/setuptools/commits/cf565b6
-        if get_distribution("setuptools").parsed_version < parse_version("18.4"):
-            self.test_args = []
-            self.test_suite = True
-
-    def run_tests(self):
-        # import here, cause outside the eggs aren't loaded
-        import pytest
-
-        errno = pytest.main(self.pytest_args)
-        sys.exit(errno)
-
-
 setup(
     author="numberly",
     classifiers=[
@@ -60,9 +34,13 @@ def run_tests(self):
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
+        "Programming Language :: Python :: 3.11",
+        "Programming Language :: Python :: 3.12",
+        "Programming Language :: Python :: 3.13",
+        "Programming Language :: Python :: 3.14",
         "Topic :: Software Development :: Libraries :: Python Modules",
     ],
-    cmdclass={"test": PyTest},
     description="Interact with Mattermost incoming webhooks easily.",
     download_url="https://github.com/numberly/matterhook/tags",
     include_package_data=True,
@@ -72,8 +50,7 @@ def run_tests(self):
     name="matterhook",
     packages=find_packages(),
     platforms="any",
-    tests_require=["pytest"],
     url="https://github.com/numberly/matterhook",
-    version="0.2",
+    version="0.3",
     zip_safe=True,
 )
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/test_attachments.py b/tests/test_attachments.py
new file mode 100644
index 0000000..aadf1c9
--- /dev/null
+++ b/tests/test_attachments.py
@@ -0,0 +1,137 @@
+from matterhook.attachments import Attachment
+
+
+class TestAttachmentInit:
+    def test_minimal(self):
+        att = Attachment(fallback="fallback text")
+        assert att.fallback == "fallback text"
+        assert att.color is None
+        assert att.pretext is None
+        assert att.text is None
+        assert att.author_name is None
+        assert att.author_link is None
+        assert att.author_icon is None
+        assert att.title is None
+        assert att.title_link is None
+        assert att.fields is None
+        assert att.image_url is None
+        assert att.thumb_url is None
+
+    def test_all_params(self):
+        fields = [{"short": True, "title": "F", "value": "V"}]
+        att = Attachment(
+            fallback="fb",
+            color="#FF0000",
+            pretext="pre",
+            text="body",
+            author_name="author",
+            author_link="https://author.com",
+            author_icon="https://author.com/icon.png",
+            title="Title",
+            title_link="https://title.com",
+            fields=fields,
+            image_url="https://image.png",
+            thumb_url="https://thumb.png",
+        )
+        assert att.fallback == "fb"
+        assert att.color == "#FF0000"
+        assert att.pretext == "pre"
+        assert att.text == "body"
+        assert att.author_name == "author"
+        assert att.author_link == "https://author.com"
+        assert att.author_icon == "https://author.com/icon.png"
+        assert att.title == "Title"
+        assert att.title_link == "https://title.com"
+        assert att.fields == fields
+        assert att.image_url == "https://image.png"
+        assert att.thumb_url == "https://thumb.png"
+
+
+class TestAttachmentPayload:
+    def test_minimal_payload(self):
+        att = Attachment(fallback="fb")
+        assert att.payload == {"fallback": "fb"}
+
+    def test_full_payload(self):
+        fields = [{"short": True, "title": "F", "value": "V"}]
+        att = Attachment(
+            fallback="fb",
+            color="#FF0000",
+            pretext="pre",
+            text="body",
+            author_name="author",
+            author_link="https://author.com",
+            author_icon="https://author.com/icon.png",
+            title="Title",
+            title_link="https://title.com",
+            fields=fields,
+            image_url="https://image.png",
+            thumb_url="https://thumb.png",
+        )
+        assert att.payload == {
+            "fallback": "fb",
+            "color": "#FF0000",
+            "pretext": "pre",
+            "text": "body",
+            "author_name": "author",
+            "author_link": "https://author.com",
+            "author_icon": "https://author.com/icon.png",
+            "title": "Title",
+            "title_link": "https://title.com",
+            "fields": fields,
+            "image_url": "https://image.png",
+            "thumb_url": "https://thumb.png",
+        }
+
+    def test_partial_payload_only_color(self):
+        att = Attachment(fallback="fb", color="#00FF00")
+        assert att.payload == {"fallback": "fb", "color": "#00FF00"}
+
+    def test_partial_payload_only_text(self):
+        att = Attachment(fallback="fb", text="hello")
+        assert att.payload == {"fallback": "fb", "text": "hello"}
+
+    def test_partial_payload_author_fields(self):
+        att = Attachment(
+            fallback="fb",
+            author_name="a",
+            author_link="https://a.com",
+            author_icon="https://a.com/i.png",
+        )
+        assert att.payload == {
+            "fallback": "fb",
+            "author_name": "a",
+            "author_link": "https://a.com",
+            "author_icon": "https://a.com/i.png",
+        }
+
+    def test_partial_payload_title_fields(self):
+        att = Attachment(
+            fallback="fb", title="T", title_link="https://t.com"
+        )
+        assert att.payload == {
+            "fallback": "fb",
+            "title": "T",
+            "title_link": "https://t.com",
+        }
+
+    def test_partial_payload_image_fields(self):
+        att = Attachment(
+            fallback="fb",
+            image_url="https://img.png",
+            thumb_url="https://thumb.png",
+        )
+        assert att.payload == {
+            "fallback": "fb",
+            "image_url": "https://img.png",
+            "thumb_url": "https://thumb.png",
+        }
+
+    def test_partial_payload_pretext(self):
+        att = Attachment(fallback="fb", pretext="before")
+        assert att.payload == {"fallback": "fb", "pretext": "before"}
+
+    def test_partial_payload_fields(self):
+        fields = [{"short": False, "title": "X", "value": "Y"}]
+        att = Attachment(fallback="fb", fields=fields)
+        assert att.payload == {"fallback": "fb", "fields": fields}
diff --git a/tests/test_incoming.py b/tests/test_incoming.py
new file mode 100644
index 0000000..2b4bb8b
--- /dev/null
+++ b/tests/test_incoming.py
@@ -0,0 +1,191 @@
+from unittest.mock import patch, MagicMock
+
+import pytest
+
+from matterhook.incoming import Webhook, InvalidPayload, HTTPError
+
+
+@pytest.fixture
+def webhook():
+    return Webhook("https://mattermost.example.com", "abc123")
+
+
+@pytest.fixture
+def full_webhook():
+    return Webhook(
+        "https://mattermost.example.com",
+        "abc123",
+        channel="town-square",
+        icon_url="https://example.com/icon.png",
+        username="bot",
+        attachments=[{"fallback": "test"}],
+    )
+
+
+class TestWebhookInit:
+    def test_minimal(self, webhook):
+        assert webhook.url == "https://mattermost.example.com"
+        assert webhook.api_key == "abc123"
+        assert webhook.channel is None
+        assert webhook.icon_url is None
+        assert webhook.username is None
+        assert webhook.attachments is None
+
+    def test_full(self, full_webhook):
+        assert full_webhook.channel == "town-square"
+        assert full_webhook.icon_url == "https://example.com/icon.png"
+        assert full_webhook.username == "bot"
+        assert full_webhook.attachments == [{"fallback": "test"}]
+
+
+class TestIncomingHookUrl:
+    def test_formats_url(self, webhook):
+        assert (
+            webhook.incoming_hook_url
+            == "https://mattermost.example.com/hooks/abc123"
+        )
+
+
+class TestSend:
+    @patch("matterhook.incoming.requests.post")
+    def test_send_simple_message(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        webhook.send("hello")
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={"text": "hello"},
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_send_with_all_params(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        webhook.send(
+            "hello",
+            channel="general",
+            icon_url="https://icon.png",
+            username="mybot",
+            attachments=[{"fallback": "fb"}],
+        )
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={
+                "text": "hello",
+                "channel": "general",
+                "icon_url": "https://icon.png",
+                "username": "mybot",
+                "attachments": [{"fallback": "fb"}],
+            },
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_send_uses_defaults(self, mock_post, full_webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        full_webhook.send("hello")
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={
+                "text": "hello",
+                "channel": "town-square",
+                "icon_url": "https://example.com/icon.png",
+                "username": "bot",
+                "attachments": [{"fallback": "test"}],
+            },
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_send_params_override_defaults(self, mock_post, full_webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        full_webhook.send(
+            "hello",
+            channel="off-topic",
+            icon_url="https://other.png",
+            username="other",
+            attachments=[{"fallback": "other"}],
+        )
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={
+                "text": "hello",
+                "channel": "off-topic",
+                "icon_url": "https://other.png",
+                "username": "other",
+                "attachments": [{"fallback": "other"}],
+            },
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_send_no_message(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        webhook.send()
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={"text": None},
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_send_raises_http_error(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(
+            status_code=500, text="Internal Server Error"
+        )
+        with pytest.raises(HTTPError, match="Internal Server Error"):
+            webhook.send("hello")
+
+    @patch("matterhook.incoming.requests.post")
+    def test_send_raises_http_error_404(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(
+            status_code=404, text="Not Found"
+        )
+        with pytest.raises(HTTPError, match="Not Found"):
+            webhook.send("hello")
+
+
+class TestSetItem:
+    @patch("matterhook.incoming.requests.post")
+    def test_setitem_string_payload(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        webhook["general"] = "hello"
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={"text": "hello"},
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_setitem_dict_payload(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        webhook["general"] = {"text": "hello", "username": "mybot"}
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={"text": "hello", "username": "mybot"},
+        )
+
+    @patch("matterhook.incoming.requests.post")
+    def test_setitem_dict_with_icon_url(self, mock_post, webhook):
+        mock_post.return_value = MagicMock(status_code=200)
+        webhook["general"] = {
+            "text": "hello",
+            "icon_url": "https://icon.png",
+        }
+        mock_post.assert_called_once_with(
+            "https://mattermost.example.com/hooks/abc123",
+            json={"text": "hello", "icon_url": "https://icon.png"},
+        )
+
+    def test_setitem_dict_missing_text(self, webhook):
+        with pytest.raises(InvalidPayload, match='missing "text" key'):
+            webhook["general"] = {"username": "bot"}
+
+
+class TestExceptions:
+    def test_invalid_payload_is_exception(self):
+        assert issubclass(InvalidPayload, Exception)
+
+    def test_http_error_is_exception(self):
+        assert issubclass(HTTPError, Exception)
+
+    def test_invalid_payload_message(self):
+        err = InvalidPayload("test error")
+        assert str(err) == "test error"
+
+    def test_http_error_message(self):
+        err = HTTPError("server error")
+        assert str(err) == "server error"
diff --git a/tests/test_init.py b/tests/test_init.py
new file mode 100644
index 0000000..1755fe5
--- /dev/null
+++ b/tests/test_init.py
@@ -0,0 +1,21 @@
+import matterhook
+from matterhook import Webhook, Attachment
+from matterhook.incoming import Webhook as IncomingWebhook
+from matterhook.attachments import Attachment as AttachmentsAttachment
+
+
+class TestModuleExports:
+    def test_webhook_exported(self):
+        assert Webhook is IncomingWebhook
+
+    def test_attachment_exported(self):
+        assert Attachment is AttachmentsAttachment
+
+    def test_all_contains_webhook(self):
+        assert "Webhook" in matterhook.__all__
+
+    def test_all_contains_attachment(self):
+        assert "Attachment" in matterhook.__all__
+
+    def test_all_length(self):
+        assert len(matterhook.__all__) == 2