ReactOS is an advanced free open source operating system providing a ground-up implementation of a Microsoft Windows® XP compatible operating system. ReactOS aims to achieve complete binary compatibility with both applications and device drivers meant for NT and XP operating systems, by using a similar architecture and providing a complete and equivalent public interface.

ReactOS is the most complete working model of a Windows® like operating system available. Consequently, working programmers will learn a great deal by studying ReactOS source code and even participating in ReactOS development.