During the past year, dramatic progress has been achieved in our understanding of Drosophila immune reactions. The completion of the Drosophila genome sequencing project, microarray analysis and the use of genetic screens have led to the identification of several new genes required to combat microbial infection, filling in some important gaps in the understanding of innate immunity. At the same time, this insect was used as a model for the study of host-pathogen interactions. The recent major advances on the mechanisms by which this insect defends itself against intrusion of pathogens are discussed in this review.