This book constitutes the refereed proceedings of the Third International Conference on COTS-Based Software Systems, ICCBSS 2004, held in Redondo Beach, CA, USA, in February 2004. The 27 revised papers presented together with summaries of workshops, panels, and tutorials were carefully reviewed and selected from 57 submissions. The papers address all current issues on commercial-off-the-shelf based software systems, from the point of view of research and development as well as from the practitioner's point of view and spanning the entire software life cycle.