The DJs and laptop performers of electronic dance music use preexistent elements such as records and digital samples to create extended improvisations. Analysis of these technologically mediated performances reveals a complex dynamic in which a modular approach to musical structure enables flexible adaptation and transformation of seemingly 'fixed' products in live contexts. This book covers the following topics: ontology; interface design and liveness in performance; the interaction of the preexistent and the novel within improvisation; and musical design as performative technology.