diff --git a/requirements_dev.txt b/requirements_dev.txt
new file mode 100644
index 0000000..2abf170
--- /dev/null
+++ b/requirements_dev.txt
@@ -0,0 +1,3 @@
+requests~=2.32.3
+python-dotenv~=1.0.1
+pytest~=8.3.5
diff --git a/test/__init__.py b/test/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/test/test_superfaktura_api.py b/test/test_superfaktura_api.py
new file mode 100644
index 0000000..2bf96b5
--- /dev/null
+++ b/test/test_superfaktura_api.py
@@ -0,0 +1,68 @@
+import os
+import pytest
+import requests
+from unittest.mock import patch, mock_open
+from superfaktura.superfaktura_api import (
+    SuperFakturaAPI,
+    SuperFakturaAPIException,
+    SuperFakturaAPIMissingCredentialsException,
+)
+
+@pytest.fixture
+def api():
+    with patch.dict(os.environ, {
+        "SUPERFAKTURA_API_KEY": "test_key",
+        "SUPERFAKTURA_API_URL": "https://api.superfaktura.cz",
+        "SUPERFAKTURA_API_EMAIL": "test_email",
+        "SUPERFAKTURA_API_COMPANY_ID": "test_company_id"
+    }):
+        return SuperFakturaAPI()
+
+def test_missing_credentials():
+    with patch.dict(os.environ, {}, clear=True):
+        with pytest.raises(SuperFakturaAPIMissingCredentialsException):
+            SuperFakturaAPI()
+
+def test_get(api):
+    with patch("requests.get") as mock_get:
+        mock_get.return_value.status_code = 200
+        mock_get.return_value.json.return_value = {"data": "test"}
+        response = api.get("test_endpoint")
+        assert response == {"data": "test"}
+
+def test_get_failure(api):
+    with patch("requests.get") as mock_get:
+        mock_get.return_value.status_code = 404
+        with pytest.raises(SuperFakturaAPIException):
+            api.get("test_endpoint")
+
+def test_download(api):
+    with patch("requests.get") as mock_get:
+        mock_get.return_value.status_code = 200
+        mock_get.return_value.content = b"test_content"
+        with patch("builtins.open", mock_open()) as mock_file:
+            with open("test_file", "wb") as f:
+                api.download("test_endpoint", f)
+            mock_file().write.assert_called_once_with(b"test_content")
+
+def test_download_failure(api):
+    with patch("requests.get") as mock_get:
+        mock_get.return_value.status_code = 404
+        with patch("builtins.open", mock_open()) as mock_file:
+            with open("test_file", "wb") as f:
+                with pytest.raises(SuperFakturaAPIException):
+                    api.download("test_endpoint", f)
+
+def test_post(api):
+    with patch("requests.post") as mock_post:
+        mock_post.return_value.status_code = 200
+        mock_post.return_value.json.return_value = {"data": "test"}
+        response = api.post("test_endpoint", '{"name": "Example"}')
+        assert response == {"data": "test"}
+
+def test_post_failure(api):
+    with patch("requests.post") as mock_post:
+        mock_post.return_value.status_code = 404
+        mock_post.return_value.json.return_value = {"error": "not found"}
+        with pytest.raises(SuperFakturaAPIException):
+            api.post("test_endpoint", '{"name": "Example"}')
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