Natural killer (NK) cells participate in innate and adaptive immune responses to obligate intracellular pathogens and malignant tumors. Two major NK cell subsets have been identified in humans: CD56(dim) CD16+ and CD56(bright) CD16-. Resting CD56(dim) CD16+ NK cells express CXCR1, CXCR2, CXCR3, CXCR4, and CX3CR1 but no detectable levels of CC chemokine receptors on the cell surface. They migrate vigorously in response to CXCL12 and CXC3L1. In contrast, resting CD56(bright) CD16- NK cells express little CXCR1, CXCR2, and CXC3R1 but high levels of CCR5 and CCR7. Chemotaxis of CD56(bright) CD16- NK cells is stimulated most potently by CCL19, CCL21, CXCL10, CXCL11, and CXCL12. Following activation, NK cells can migrate in response to additional CC and CXC chemokines. Cytolytic activity of NK cells is augmented by CCL2, CCL3, CCL4, CCL5, CCL10, and CXC3L1. Moreover, proliferation of CD56(dim) CD16+ NK cells is costimulated by CCL19 and CCL21. Activated NK cells produce XCL1, CCL1, CCL3, CCL4, CCL5, CCL22, and CXCL8. Chemokines secreted by NK cells may recruit other effector cells during immune responses. Furthermore, CCL3, CCL4, and CCL5 produced by NK cells can inhibit in vitro replication of HIV. CCL3 and CXL10 expression appear to be required for protective NK cell responses in vivo to murine cytomegalovirus or Leishmania major, respectively. Moreover, NK cells participate in the in vivo rejection of transduced tumor cells that produce CCL19 or CCL21. Thus, chemokines appear to play an important role in afferent and efferent NK cell responses to infected and neoplastic cells.