The development of vaccines and also of methods for the monitoring of cytotoxic T lymphocyte (CTL) responses depends on the identification of epitopes from immunologically relevant antigens. Detailed information about the rules that govern the interactions of peptides with MHC class I molecules, together with an increasing knowledge about the cleavage specificities of proteasomes, has paved the way for the development of computer programs that can predict CTL epitopes. The combination of such programs is likely to speed up the identification of CD8+ T cell epitopes relevant for immune responses to infectious diseases and cancer.