The regulation of an adaptive T cell immune response involves the tuning of multiple costimulatory proteins that modulate the specific antigen-recognition signal directed through the T cell receptor. While immune checkpoint proteins are crucial for maintaining tolerance, chronic exposure to antigen as experienced in viral infections and cancer leads to a state of functional exhaustion for T cells, characterized by anergy and loss of effector cytolytic and cytokine activity. Blockade of the accumulated checkpoint inhibitors restores effector T cell and NK cell function and reduces the immunosuppressive response of T regulatory cells. The recent clinical success of checkpoint inhibitor antibodies blocking the CTLA-4 and PD-1 pathways has led to a rapid growth in the field of cancer immunotherapy. While antibodies against many of the major players are rapidly moving to the clinic, the field still lacks a detailed understanding of the mechanisms of blockade function, the intracellular signaling pathways of the known inhibitors, and the physiologically relevant ligand(s) for a number of these molecules. Our group aims to better understand these pathways through defining the roles of ligand blocking in murine models of cancer as well as the relative contribution of checkpoint expression on multiple cells types (i.e., T cells, NK cells, dendritic cells). Untangling the complex biology of checkpoint expression and interactions will not only add to our understanding of these basic tolerance mechanisms but also inform clinical development and strategy in the area of immunotherapy.