Information regarding the role of natural killer (NK) cells in the response to viruses, intracellular bacteria and parasites continues to emerge. NK cells can directly lyse infected cells, secrete cytokines and interact with dendritic cells to drive the adaptive immune response. There are a large number of activating and inhibitory receptors that govern NK cell activity. Recent studies have revealed how signals are transmitted and integrated from the variety of receptors, how particular receptors influence NK development and functional status, and how NK cells access lymph nodes and sites of infection. The potential for NK cells to exhibit specific and memory-like responses has begun to blur the 'innate' definition of NK cells.