is an operating system (OS for short) that is binary compatible with most Microsoft Windows programs, features, and services. It is written mostly in C, with some parts, such as the File Explorer, in C++ and some minor bits of assembly. It is mostly licensed under the GNU General Public License, similar to the Linux kernel. ReactOS is built from reverse engineering portions of Windows XP and Windows Server 2003. Because it is mostly binary compatible with Windows, many users choose it as an alternative to Linux while staying within the realm of free and open source software. Because of the method used to build ReactOS, it's developers claim that their work is entirely original, and that no code is derived from Windows - all developers who work on ReactOS must either write original code, or create documentation based on decompiled binaries of Windows, but not both, in order to comply with American laws on reverse engineering.

The ReactOS kernel is a re-implementation of Windows' ntoskrnl.exe, for Windows Server 2003. It has been combined with a port of the WINE project to provide a userland for the operating system, which is mostly identical to the standard userland available on Windows. As a result, ReactOS is able to run any programs which WINE can run, and WINE has access to what the internals of Windows probably look like, advancing both projects to provide support for Windows programs on UNIX-like systems, such as Microsoft's Xenix, GNU\Linux, FreeBSD, and other operating systems. ReactOS also has limited support for DirectX 9, allowing some games available for Windows XP to run normally under ReactOS without modification.

ReactOS is available for x86 processors, as well as some work in progress ports such as x86-64 processors, ARM, and PowerPC.


The project started in 1996 with the goal to create a Windows 95 clone called "FreeWin95". Not much happened during this time and the project was eventually halted when Windows 98 was released. The project was then rebooted in 1998 by Jason Filby with its new goal of being a Windows NT clone. The name, ReactOS, comes from the developers reaction to Microsofts monopoly over the operating system market.

The first years of ReactOS were slow as only a few people were working on the project who has knowledge of kernel code. As more drivers were made and as ReactOS grew more stable, more developers joined the project. ReactOS also participates in Google Summer of Code and has been accepted in 2006, 2011, 2016, 2017, and 2018, of which some of the students decided to stay with the project.

Recent Changes

The current version is 0.4.11. This version, most notable as one of the largest updates to date, brings on many useful changes. First, the ReactOS team has completely finished it's GSoC 2018 goal of WinBTRFS read/write support and is now able to fully boot from a WinBTRFS partition. Some major work has been done on 1st stage GUI setup and is now able to detect other filesystems that work in ReactOS, as well as being able to upgrade/repair existing partitions. Full Bidirectional support has been implemented for right to left languages. The GSoC 2016/2017 USB drivers for UHCI and EHCI have been integrated, but are currently disabled by default for testing purposes only. Lastly, there has been many changes to Win32SS, such as Ghosting Windows for hanging applications, improvements in font penalty to help makes fonts render correctly, some fixes to pen drawing, and many more bug fixes.

System Requirements

  • 500MB IDE or SATA drive.
  • 96MB ram.
  • PS/2 or USB keyboard and mouse.
  • VGA or VESA display.

Filesystem Support

Currently, ReactOS can read, write and boot from FAT32 and WinBTRFS. NTFS is still work in progress with full read and only partial write support. Read and mount support exist for ReiserFS and Ext2/3/4.


ReactOS Website