Natural killer cells were first identified by their cytotoxic activity against tumor cells, suggesting a role in immunological surveillance against neoplasia. However, there is now increasing evidence that natural killer cells are important mediators of innate resistance against a variety of pathogenic micro-organisms. Recently, several important advances have been made in our understanding of how these cells are activated during infection, the contribution of cytokines derived from natural killer cells to host resistance and their influence on the development of antigen-specific T-cell responses.