Lexikon-Sonate is an interactive realtime composition environment for musical composition and live performances. It takes advantage of composition algorithms that has been developed by Karlheinz Essl since 1985. With this algorithmic music generator on can create fascinating and complex musical structures on the fly. Furthermore, Lexikon-Sonate is an infinite music installation that can run on a computer for years without repeating itself. Finally, Lexikon-Sonate can be used as an instrument for live performance of algorithmic piano music.