Abstract: The immune system is an important defense mechanism against cancer and is often dysfunctional in patients with malignancies. The central regulator of the anti-cancer adaptive immune response is the T lymphocyte. T lymphocyte activation requires the completion of a carefully orchestrated series of specific steps that can be preempted or disrupted by any number of critical events. Particularly important is the provision of a costimulatory signal, the binding of accessory molecules on the antigen presenting cell to receptors on the T lymphocyte. Though costimulatory signals were traditionally envisioned as T lymphocyteactivating events, recent discoveries have highlighted their duality: they can be either stimulatory (costimulation) or inhibitory (coinhibition). In this article we review costimulation and coinhibition as potential targets for cancer therapy. We begin by presenting a general framework for thinking about the immune system in the context of cancer. Our discussion then bridges the various aspects of immune dysfunction seen in cancer with the presence of coinhibitory (ex: PD-1, PD-L1, CTLA-4, BTLA) and costimulatory (ex: CD28, ICOS, 4-1BB, CD40, OX40, CD27) signaling. Lastly, we develop a model of cancerrelated immune dysfunction that parallels the concept of immunoediting. Throughout the article we emphasize clinically relevant research often applicable-but not limited-to the example of renal cell carcinoma.