PD-1 or PDCD1 (Programmed cell death 1), also known as CD279, is a cell surface receptor expressed on T cells and pro-B cells, which was firstly isolated from a murine T cell hybridoma and a hematopoietic progenitor cell line in 1992. It is an inducible immune modulatory receptor and belongs to a member of the CD28 receptor family. The expression of PD-1 is significantly involved in programmed cell death of lymphocytes. PD-1 has two ligands that are PD-L1 (B7-H1 or CD274) and PD-L2 (B7-DC or CD273). Upon their interactions, PD-1 plays an active role in negative regulation of T cell responses to antigen stimulation and maintaining peripheral tolerance. Thus, it is regarded as an important immune checkpoint for therapeutic antibodies against cancers. TIM-3 (T cell immunoglobulin and mucin domain 3) has been shown to mediate immune tolerance in mouse models of infectious diseases, alloimmunity, autoimmunity, and tumor Immunity. Thus, targeting TIM-3 emerges as a promising approach for further improvement of current immunotherapy. Despite a large amount of experimental data showing an immune suppressive function of TIM-3 in vivo, the exact mechanisms are not well understood. To enable effective targeting of TIM-3 for tumor immunotherapy, further in-depth mechanistic studies are warranted. These studies will also provide much-needed insight for the rational design of novel combination therapy with other checkpoint blockers. In this review, we summarize key evidence supporting an immune regulatory role of TIM-3 and discuss possible mechanisms of action.