We present an interactive system, Mixxx, for live DJ'ing using digital sound files. The design of the system is approached from two directions: Through Contextual Design using contextual interviews and video recordings and Open Source development where feedback and ideas are generated by developers and users from the open source community. Our contextual interviews show that DJs use a significant amount of their time on tracking and synchronizing songs using the traditional setup with turntables or CD players. By making beat information an integrated part of our DJ software Mixxx, synchronization is done automatically and DJs can use more time to attend other parts of the mix. We provide an intuitive interface for mixing with beat information that allows the same level of flexibility as with the traditional setup but facilitates new creative ways of mixing. Categories & Subject Descriptors: H.5.5. Information Interfaces and Presentation: Sound and Music Computing. General Ter...