Hercules is an open source software implementation of the mainframe System/370 and ESA/390 architectures, in addition to the latest 64-bit z/Architecture. Hercules runs under Linux, Windows, Solaris, FreeBSD, and Mac OS X.

Hercules is OSI Certified Open Source Software licensed under the terms of the Q Public Licence.

The Hyperion version of Hercules is the official development version of the Hercules emulator and contains the latest bleeding edge changes made by Hercules developers to address various bugs that may exist in the production version.

Hercules was created by Roger Bowler. Jay Maynard "the Tron Guy" was the maintainer from 2000 to 2012. Jan Jaeger designed and implemented many of the advanced features of Hercules, including dynamic reconfiguration, integrated console, interpretive execution and z/Architecture support. A dedicated crew of programmers is constantly at work implementing new features and fixing bugs.