The research project "Information Structures in Music" applies methods used in computer science to analyze and model musical structures. One research interest concentrates on rule-based techniques (e.g. formal grammars), another uses learn-based approaches, especially artificial neural networks and genetic algorithms. For both approaches we present a system solving the musical task of  chorale harmonization. The resulting music pieces are judged by experts to be of similar quality as those invented by an improvising organist.