ABSTRACT Earlier interactive improvisation systems have mostly worked with note-level musical events such as pitch, loudness and duration. Timbre is an integral component of the musical language of many improvisers; some recent systems use timbral information in a variety of ways to enhance interactivity. This article describes the timbre-aware ARHS improvisation system, designed in collaboration with saxophonist John Butcher, in the context of recent improvisation systems that incorporate timbral information. Common practices in audio feature extraction, performance state characterization and management, response synthesis and control of improvising agents are summarized and compared.