Nowadays, powerful music compression tools and cheap mass storage devices have become widely available. This allows average consumers to transfer entire music collections from the distribution medium, such as CDs and DVDs, to their computer hard drive. To locate specific pieces of music, they are usually labeled with artist and title. Yet the user would benefit from a more intuitive organization based on music style to get an overview of the music collection. We have developed a novel tool called MUSCLE which fills this gap. While there exist approaches in the field of musical genre classification, none of them features a hierarchical classification in combination with interactive user feedback and a flexible multiple assignment of songs to classes. In this paper, we present MUSCLE, a tool which allows the user to organize large music collections in a genre taxonomy and to modify class assignments on the fly.