Because of the evolutionary conservation of innate mechanisms of host defense, Drosophila has emerged as an ideal animal in which to study the genetic control of immune recognition and responses. The discovery that the Toll pathway is required for defense against fungal infection in Drosophila was pivotal in studies of both mammalian and Drosophila immunity. Subsequent genetic screens in Drosophila to isolate additional mutants unable to induce humoral responses to infection have identified and ordered the function of components of two signaling cascades, the Toll and Imd pathways, that activate responses to infection. Drosophila blood cells also contribute to host defense through phagocytosis and signaling, and may carry out a form of self-nonself recognition that is independent of microbial pattern recognition. Recent work suggests that Drosophila will be a useful model for dissecting virulence mechanisms of several medically important pathogens.