ARJ (Archived by Robert Jung) is a software tool designed by Robert K. Jung for creating high-efficiency compressed file archives. ARJ is currently on version 2.86 for DOS and 3.20 for Windows and supports 16-bit, 32-bit and 64-bit Intel architectures.
ARJ was one of two mainstream[clarification needed] archivers for DOS and Windows during the early and mid-90s, with PKZIP being its competition. Parts of ARJ were covered by U.S. Patent 5,140,321. Generally ARJ was less popular than PKZIP, but it did enjoy a niche market during the BBS era and in the warez scene. This was largely due to ARJ's creation and handling of multi-volume archives (archives which are split into smaller files which are then suitable for dial-up transfers and floppy distribution) being more robust than PKZIP's.
ARJ compressed files with the Filename extension .arj can be unpacked with various tools other than the ARJ software. There exists a free software re-implement of the tool. The software 7-Zip and Zipeg can also unpack .arj files.  For Mac there exists a standalone utility UnArjMac.