Innate immunity, more ancient than adaptive immunity in evolution, comprises a broad array of cells and recognition strategies. In contrast to the dramatic diversity of the adaptive immune response -- B cells and T cells -- innate immunity targets less diverse features identifying microbes or diseased cells. Examples are cell wall components or other defining features of microorganisms, or molecules regulated by specific cell stress pathways that are activated in host cells as a result of infection or transformation. In invertebrates, innate immunity is the sole form of immune defense and yet can be quite effective. In vertebrates, innate immunity provides considerable protection by itself and also has a major role in promoting and shaping adaptive immune responses. Key developments in this emergent field include the definition of innate immune receptor families in invertebrates and mammals that recognize pathogens and cancer cells, disease susceptibility in humans with specific genetic defects in innate immunity, and cytokine-mediated mechanisms that protect normal cells during the course of an innate immune response. The speakers in this symposium will highlight the roles of innate immunity in organisms as diverse as flies and humans.