Cancer immunotherapy relies on the ability of the immune system to target tumor-specific antigens to generate an immune response. This initial response requires both binding of the MHC/antigen peptide to T-cell receptor complex, along with a second costimulatory signal created by the binding of CD28 on the T cell, with B7 located on the antigen-presenting cell. Regulatory checkpoints, such as cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), serve to attenuate this signal, thereby preventing autoimmunity. Its key role in regulating the immune system has made CTLA-4 an attractive therapeutic target for cancer, with the development of fully human monoclonal antibodies that have successfully targeted CTLA-4 in clinical trials. Augmentation of the immune response via blockade of CTLA-4 represents a significant advance in the field of oncology and has shown an improvement in survival for patients with metastatic melanoma. An increased understanding of the components of this pathway and the identification of other methods to modulate the immune system hold great promise for future therapy.