Antigens presented by class I of the major histocompatibility complex (MHC) are recognised by the T cell receptor of CD8+ cytolytic effector cells (CTLs), while class II molecules present antigens to CD4+ helper T cells. For both class I and class II molecules, structure and function are linked through the binding of peptides. Consensus or individual sequences have been obtained for naturally processed peptides bound to a variety of class I and class II molecules, revealing the general features of peptides associated with MHC molecules. The interactions between peptides and MHC molecules have been more clearly defined by the characterization of the three dimensional structure of several different MHC molecules. CTLs have been implicated in immune responses against tumors and it is now well documented that some human tumors express specific antigens, which are recognised by CTLs and could potentially be used in immunotherapy protocols. The use of antigenic peptides to elicit a specific and effective CTL response in vivo offers several advantages over the use of other antigenic moieties. Emerging strategies for the safe and effective administration of peptides to humans may lead to their use in the immunological prevention and treatment of cancer.