"Open source Windows" ReactOS is now 30 years old

ReactOS is a rather interesting open-source project that is considered by many to be a direct and free drop-in replacement for Windows, especially if you don't want to be locked in to Microsoft's proprietary ecosystem. One of its goals is to allow customers to run Windows apps and drivers in an open-source environment (reminiscent of Windows XP) that they can trust, and it has made many advancements in this regard. It supports Microsoft's FAT file system, Registry caching, native .zip handling, and is even capable of running Microsoft's iconic Hover! in fullscreen. Now, the project is celebrating its 30th anniversary.

In a commemorative blog post, ReactOS' Core Developer Carl Bialorucki has shed some light on the history of the OS, noting that the first commit to its source tree was made 30 years ago this day.

Between 1996 and 2003, ReactOS was still finding its footing after the stalling of the FreeWin95 project, which aimed to implement a clone of Windows 95. ReactOS changed this direction a bit and set out to build a Windows NT-like kernel, after which drivers were written for it. Eric Kohl, the person who wrote the original storage driver stack for ReactOS had the following to say about the initiative's early days:

I think I found ReactOS while searching for example code for my contributions to the WINE project. I subscribed to the mailing list and followed the discussions for a few days. The developers were discussing the future of shell.exe, a little command line interpreter that could only change drives and directories and execute programs. A few days [later] I had started to convert the FreeDOS command.com into a Win32 console application, because I wanted to extend it to make it 4DOS compatible. 4DOS was a very powerful command line interpreter. On December 4th, 1998 I introduced myself and suggested to use my converted FreeDOS command.com as the future ReactOS cmd.exe. I had a little conversation with Jason Filby and Rex Joliff, the CVS repository maintainer. I sent my cmd.exe code to Rex and he applied it to the repository. After applying a few more cmd-related patches over the next weeks, Rex asked me whether I would like to have write-access to the repository. I accepted the offer.

 

[...] There was always an open and friendly atmosphere. It was and still is always nice to talk to other developers. No fights, no wars, like in some other projects.

ReactOS finally reached version 0.1.0 in February 2003 with a command line interface (CLI) that was bootable from a CD. You can see it in action below:

2003-2006 was a tricky period where there were advancements in development through a new desktop GUI and new drivers, but contributions had to be frozen after concerns grew about contributors using proprietary Windows code in ReactOS. This prompted a full audit of the existing code, as negligence could lead to a lawsuit from Microsoft.

Version 0.3.x was worked on between 2006-2016, which saw the introduction of networking support, a package manager, and the UniATA driver to support SATA storage devices and partitions greater than 8GB in size.

This brings us to ReactOS 0.4.x today which has support for kernel debugging using WinDbg when compiled with MSVC, a new graphical shell that is very similar in architecture to Windows Explorer, and more.

Despite being in development for 30 years at this point, interest in the project hasn't waned. In terms of what's next, Bialorucki has talked about several out of tree projects which may be integrated into ReactOS at some point. These include a new build environment for developers, new NTFS and ATA drivers, kernel and usermode address space layout randomization (ASLR), support for modern GPU drivers built on WDDM, and a lot more. If this piques your interest, you can financially contribute to ReactOS here or collaborate on GitHub here.

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Posted Friday 23 January 2026 at 5:47 pm AEST (my time).

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