PD-1 (Programmed Death-1 receptor) is a key regulator of the threshold of immune response and peripheral immune tolerance. It is expressed on activated T cells, B cells, monocytes, and dendritic cells and binds to PD-L1 or PD-L2. PD-1 ligation induces co-inhibitory signals in T cells promoting their apoptosis, anergy, and functional exhaustion. Blockade of the PD-1: PD-L1 interaction enhances anti-tumor immunity and shows potential for improving cancer immunotherapy.