NK (natural killer) cells are large granular lymphocytes of the innate immune system. They are widespread throughout the body, being present in both lymphoid organs and nonlymphoid peripheral tissues. NK cells are involved in direct innate immune reactions against viruses, bacteria, parasites, and other triggers of pathology, such as malignant transformation, all of which cause stress in affected cells. Importantly, NK cells also link the innate and adaptive immune responses, contributing to the initiation of adaptive immune responses. NK cell responses are mediated through two major effector functions, the direct cytolysis of target cells and the production of cytokines and chemokines. We focus here on how recognition events are integrated to trigger distinct and graded effector functions.