This is the fourth meeting in the series of Keystone Symposia on Cellular Immunity and Immunotherapy of Cancer. Previous meetings, by the same organizers, were held in 1990, 1993 and 1997. These meetings are unique because each one serves as a yard stick for measuring progress in tumor immunology. No other tumor immunology meeting that is planned for the next two years has the scope, the breadth and the continuity that this meeting ensures through the Keystone Symposia venue. The three previous meetings have featured the newest developments in basic immunology and related fields that could impact on our understanding of tumor-specific immunity. This latest meeting will feature breakthroughs in tumor immunology and will also include the most recent discoveries in the areas of antigen processing and presentation, T and B cell activation, tolerance, memory and apoptosis, and their relevance to tumor-specific immunity. A special session will be devoted to lessons learned from infectious and autoimmune diseases. The newest clinical approaches to immunotherapy will also be evaluated. The program is designed to bring immunologists working on basic immune mechanisms, viral immunity and autoimmunity together with tumor immunologists and immunotherapists. As in our previous meetings, we expect to provoke and encourage intense discussions and free flow of ideas. We have paid special attention to inviting young investigators and women as plenary session speakers. Of 33 invited speakers, 7 are young investigators selected based on their publications and/or impressive presentations at other meetings. We have reserved 7 slots in the program for additional young investigators who will be selected for short plenary presentations based on submitted abstracts.