Programmed cell death-1 (PD-1) is a member of the CD28 superfamily that delivers negative signals upon interaction with its two ligands, PD-L1 or PD-L2. PD-1 and its ligands are broadly expressed and exert a wider range of immunoregulatory roles in T cells activation and tolerance compared with other CD28 family members. Subsequent studies show that PD-1–PD-L interaction regulates the induction and maintenance of peripheral tolerance and protect tissues from autoimmune attack. PD-1 and its ligands are also involved in attenuating infectious immunity and tumor immunity, and facilitating chronic infection and tumor progression. The biological significance of PD-1 and its ligand suggests the therapeutic potential of manipulation of PD-1 pathway against various human diseases. In this review, we summarize our current understanding of PD-1 and its ligands ranging from discovery to clinical significance.