Natural killer (NK) cells are innate lymphoid cells important for host defense against patho- gens, and are also thought to play an important role in the immunosurveillance against can- cers. NK cells express surface receptors that interact with target cells and determine their response to infected or transformed cells without prior sensitization or gene rearrangement. Activated NK cells can kill target cells by producing effector molecules and transmit impor- tant information to the rest of the immune system subsequently by producing cytokines. Here, we review the mechanism of cytotoxic function of NK cells and a recent development in NK cell biology, and highlight therapeutic approaches for NK cell-based immunotherapy and targeting NK cells in the treatment of cancer. //.