PD-L1, also known as B7-H1 and CD274, is an approximately 65 kDa transmembrane glycoprotein in the B7 family of immune regulatory molecules. PD-L1 is expressed on inflammatory-activated immune cells including macrophages, T cells, and B cells, keratinocytes, enothelial and intestinal epithelial cells, as well as a variety of carcinomas and melanoma. PD-L1 protein binds to T cell B7-1/CD80 and PD-1. It suppresses T cell activation and proliferation and induces the apoptosis of activated T cells. It plays a role in the development of immune tolerance by promoting T cell anergy and enhancing regulatory T cell development. PD-L1 favors the development of anti-inflammatory IL-10 and IL-22 producing dendritic cells and inhibits the development of Th17 cells. In cancer, PD-L1 provides resistance to T cell mediated lysis, enhances EMT, and enhances the tumorigenic function of Th22 cells. Many tumors overexpress PD-L1 protein making the PD-1:PD-L1 interaction central to several successful cancer immunotherapy approaches.