GREG (Guided Real-time Euphony Generator) is a procedural generation music program written in JavaScript for web applications, art games, interactive animations and other multimedia works by the author. GREG begins with a composition by the author and a set of parameters that guide randomized changes to the melody, harmony, rhythm, instrument voices and sound effects over an indefinite series of playback loops. GREG reacts to an accompanying game or animation, reflecting changes in state with changes in key, tempo, and other parameters, to match the tone or feeling of the animation. GREG enables dynamic accompanying music to animation and interactive projects with a streamlined composition process. Over several years of development, GREG has more recently become a standalone project, with the author using it as a music composition and performance tool.