Natural killer (NK) cells are one of the first lines of defense against a wide range of pathogens and cancers. They express a variety of both inhibitory and activating receptors and antigens that can be used to classify cells as a particular subset population with differing regulatory and effector functions. New studies reveal previously uncharacterized complexities of NK development, functions, roles, and responses to stimuli within these subsets of NK cells. These new characteristics include populations of NK cells that are involved in regulation, memory, hematopoiesis, positive and negative selection, and interactions between adaptive and innate cells. NK cells thus serve as an important innate, anti-pathogen and anti-tumor cell with possible adaptive immunity features.