PD-1 (programmed cell death-1) is a transmembrane protein important for regulating immune responses. It is a member of the CD28/CTLA-4 family of immunoreceptors and is expressed on T cells and pro-B cells suggesting it has a broad spectrum of immune regulation. PD-1 along with its two ligands, PD-L1 and PD-L2, prevent the activation of T cells and protect tissues from autoimmune attack. PD-1 and its ligands are also involved in reduction of infectious immunity, as well as tumor immunity. PD-1 has been shown to facilitate chronic infection and tumor progression. Drugs targeting PD-1 have shown promise in treating cancer and HIV and may be important in a variety of immune related disorders.