This document provides an overview of Windmill, an open source test automation tool for web applications. Windmill allows users to: - Run single tests across multiple browsers for cross-browser compatibility testing - Easily debug tests in an interactive way - Integrate tests into continuous integration workflows Windmill was created because existing solutions did not meet all needs such as difficult test authoring and debugging. It provides a test authoring library, IDE for writing tests, and service for executing tests across browsers. The future of Windmill includes supporting more programming languages, improving the user interface, and packaging.