diff --git a/Dockerfile b/Dockerfile
index 3a6b3e0..6e00a74 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,12 +1,37 @@
-FROM python:3.13
+# syntax=docker.io/docker/dockerfile:1.7-labs
+FROM python:3.13 AS deps
 
 WORKDIR /usr/src/app
 
+COPY dev-requirements.txt ./
+RUN pip install --no-cache-dir -r dev-requirements.txt
+RUN rm dev-requirements.txt
+
+#=======================================================
+
+FROM deps AS app
+
+WORKDIR /usr/src/app
 RUN apt-get update && apt-get install -y ffmpeg
 
 COPY requirements.txt ./
 RUN pip install --no-cache-dir -r requirements.txt
+RUN rm requirements.txt
+
+COPY --exclude=test_*.py src ./src
+
+# Thanks to https://stackoverflow.com/questions/64776990/python-docker-no-module-found
+ENV PYTHONPATH /usr/src/app
+EXPOSE 8000
+ENTRYPOINT ["python"]
+CMD ["src/main.py"]
+
+#=======================================================
+
+FROM deps AS test
+
+COPY dev-requirements.txt ./
+RUN pip install --no-cache-dir -r dev-requirements.txt
 
 COPY . .
-
-CMD [ "python", "./main.py" ]
\ No newline at end of file
+# No `CMD`, because there are plenty of potential test commands, no single sensible default.
diff --git a/compose.yaml b/compose.yaml
new file mode 100644
index 0000000..1f2087f
--- /dev/null
+++ b/compose.yaml
@@ -0,0 +1,46 @@
+services:
+  base: &base
+    tty: true
+    stdin_open: true
+    # Necessary because compose does not permit a service to lack a build
+    build:
+      context: .
+      target: deps
+    entrypoint: python
+    volumes:
+      - .:/usr/src/app:delegated
+
+  app:
+    build:
+      context: .
+      target: app
+    environment:
+      - DOWNLOAD_DIR=/download
+    ports:
+      - "8000:8000"
+    volumes:
+       - ./download_dir:/download
+
+  test-base: &test-base
+    <<: *base
+    build:
+      context: .
+      target: test
+
+  test:
+    <<: *test-base
+    command: ["-m", "pytest", "--ignore=integ-tests", "."]
+
+  test-with-output:
+    <<: *test-base
+    command: ["-m", "pytest", "-rP", "--ignore=integ-tests", "."]
+
+  test-integ:
+    <<: *test-base
+    depends_on:
+      # TODO - should really have a healthcheck here to avoid race-conditions. "Up" doesn't actually mean "Up and accepting requests"
+      - app
+    command: ["-m", "pytest", "integ-tests" ]
+    volumes:
+       - ./download_dir:/download
+       - .:/usr/src/app:delegated # Have to re-specify this because the specification above overrides, rather than appending
diff --git a/dev-requirements.txt b/dev-requirements.txt
new file mode 100644
index 0000000..154e384
--- /dev/null
+++ b/dev-requirements.txt
@@ -0,0 +1,2 @@
+pytest==8.3.4
+requests==2.28.1
\ No newline at end of file
diff --git a/download_dir/README.md b/download_dir/README.md
new file mode 100644
index 0000000..2eb21c1
--- /dev/null
+++ b/download_dir/README.md
@@ -0,0 +1 @@
+A location into which the application can download files when run locally. Particularly useful for sharing with the Integ Test service!
diff --git a/integ-tests/test_end-to-end.py b/integ-tests/test_end-to-end.py
new file mode 100644
index 0000000..5bc3b93
--- /dev/null
+++ b/integ-tests/test_end-to-end.py
@@ -0,0 +1,35 @@
+import pathlib
+import requests
+import time
+
+def test_download():
+    target_file_name = "A Beginner's Guide to the EICAR Test File [bTThnbwxN5g].m4a"
+    download_path = pathlib.Path('/download')
+    url = 'https://www.youtube.com/watch?v=bTThnbwxN5g'
+
+    try:
+        # `app` is injected as a DNS name by the Docker Compose harness
+        response = requests.post('http://app:8000/download', json={
+            'url': url
+        })
+        assert response.status_code == 202, f"Non-202 response: {response.status_code}. Body: {response.json()}"
+        time.sleep(2)
+        for _ in range(5):
+            if _does_target_file_exist(target_file_name, download_path):
+                break
+            else:
+                time.sleep(3)
+        else:
+            assert False, "File not found after 15 seconds"
+    finally:
+        for f in download_path.iterdir():
+            if f.name != 'README.md':
+                f.unlink()
+
+def _does_target_file_exist(file_name: str, dir: pathlib.Path) -> bool:
+    for f in dir.iterdir():
+        if f.name == file_name:
+            return True
+        else:
+            print(f"Found file: {f.name} which does not match")
+    return False
diff --git a/requirements.txt b/requirements.txt
index b1df116..6fccefb 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1 +1 @@
-yt-dlp
+yt-dlp==2024.10.22
diff --git a/src/__init__.py b/src/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/handler.py b/src/handler.py
similarity index 97%
rename from handler.py
rename to src/handler.py
index 31d2865..f940d98 100644
--- a/handler.py
+++ b/src/handler.py
@@ -69,7 +69,7 @@ class Handler(http.server.SimpleHTTPRequestHandler):
 
     def do_GET(self):
         self.send_response(HTTPStatus.INTERNAL_SERVER_ERROR)
-        content = 'ERROR: Only HEAD requests are permitted\n'.encode('utf-8')
+        content = 'ERROR: Only POST requests are permitted\n'.encode('utf-8')
         self.send_header("Content-type", 'application/json')
         self.send_header('Content-Length', len(content))
         self.end_headers()
diff --git a/main.py b/src/main.py
similarity index 93%
rename from main.py
rename to src/main.py
index 726bb1f..244c56c 100644
--- a/main.py
+++ b/src/main.py
@@ -4,7 +4,7 @@ import os
 import socketserver
 
 from functools import partial
-from handler import Handler
+from src.handler import Handler
 from multiprocessing import Pool
 
 PORT = int(os.environ.get('PORT', '8000'))
diff --git a/src/test_handler.py b/src/test_handler.py
new file mode 100644
index 0000000..01d3736
--- /dev/null
+++ b/src/test_handler.py
@@ -0,0 +1,20 @@
+import os
+import pathlib
+import tempfile
+
+from src.handler import download
+
+def test_download():
+    video_url = "https://www.youtube.com/watch?v=bTThnbwxN5g"
+    expected_filename = "A Beginner's Guide to the EICAR Test File [bTThnbwxN5g].m4a"
+    with tempfile.TemporaryDirectory() as tmpdirname:
+        os.environ['DOWNLOAD_DIR'] = tmpdirname
+        download(video_url)
+        download_dir = pathlib.Path(tmpdirname)
+        passes = False
+        for contents in download_dir.iterdir():
+            print(f'DEBUG - {contents=}')
+            if contents.name == expected_filename:
+                passes = True
+                break
+        assert passes, "Could not find downloaded file"