ReactOS is a computer operating system intended to be binary compatible with application software and device drivers made for Microsoft Windows NT versions 5.x and up (Windows 2000 and its successors). It is claimed to be composed entirely of free software, by means of a complete clean room reverse engineering process, with an extensive on-going audit, being undertaken to protect against claims made by those outside the project.
Although the project is in the alpha development stage as of 2009, many Windows programs already work well. While the ReactOS kernel has been written from scratch, the userland is partially based on the Wine compatibility layer for Unix-like operating systems.
ReactOS is primarily written in the C programming language. Various components of ReactOS are licensed under the GNU General Public License, the GNU Lesser General Public License and the BSD License.