Reason is a music software program developed by Swedish software developers Propellerhead Software. It emulates a rack of hardware synthesizers, samplers, signal processors, sequencers and mixers, all of which can be freely interconnected in an arbitrary manner. Reason can be used either as a complete virtual music studio, or as a collection of virtual instruments to be played live or used with other sequencing software.

Reason 5 was released on 25 August 2010. For the first time, Reason can directly sample audio with any of its sample-playing instruments such as the Redrum, the NN-19, the NN-XT, and the Kong Drum Designer.

The Dr. Rex loop player has been given an overhaul and is now called \"Dr. Octo Rex\" and can play up to eight sample loops (one at a time), and includes many new features for editing the loops and individual clips.

New to the program is the "Kong Drum Designer," a 16-pad device that can create drum and percussion sounds using various sound production techniques, including physical modeling, sample playback (the "NN-Nano" module), and virtual analog synthesis.

Signature Patches created by well known Reason users have been added, including patches made by Two Lone Swordsmen, Vengeance and Richard Barbieri.

Finally, a new pattern editing tool called "Blocks" has been created for easier song arrangement and mixing.