Tumor immunotherapy harnesses the potential of the host immune system to recognize and eradicate neoplastic tissue. The efficiency of the immune system in mediating tumor regression depends on the induction of antigen-specific T-cell responses through physiologic immune surveillance, priming by vaccination, or following adoptive transfer of T-cells. Although a variety of tumor-associated antigens have been identified and many immunotherapeutic strategies have been tested, objective clinical responses are rare. The reasons for this include the inability of current immunotherapy approaches to generate efficient T-cell responses, the presence of regulatory cells that inhibit T-cell responses, and other tumor escape mechanisms. The activation of effector T-cells depends on interactions between the T-cell receptor (TCR) and cognate antigen presented as peptides within the major histocompatibility complex (MHC) and costimulatory signals delivered by CD28, which binds to B7.1 and B7.2. More recently, several new molecular receptors and ligands have been identified that integrate into stimulatory or inhibitory activity for T-cells. These signals have been loosely associated with the costimulatory molecules but actually represent a diverse group of molecular pathways that have unique and overlapping functions. This review will focus on these pathways and emphasize their role in mediating T-cell activation for the purpose of enhancing tumor immunotherapy. As we gain a better understanding of the molecular and cellular consequences of T-cell signaling through the costimulatory pathways, a more rational approach to the activation or inhibition of T-cell responses can be developed for the treatment of cancer and other immune-mediated diseases.