Insects have developed an efficient host-defense against microbial infections that relies on several innate reactions: (1) induction upon injury of two proteolytic cascades leading to localized blood clotting and to melanisation; (2) cellular response that includes phagocytosis and encapsulation of invaders and which is predominant in the ancient insect orders; (3) the rapid and transient synthesis of a battery of antimicrobial peptides, a hallmark of the immune response in phylogenetically recent insect orders. Over the last two decades, tremendous progress has been made regarding the characterization of insect antimicrobial peptides as well as the analysis of their gene expression. Since the discovery of the first antibacterial peptide cecropin by Boman and associates in the early 1980's, more than 100 inducible peptides have been identified. Considering the vast diversity of insect species and the fact that only a small fraction has been examined to date, one predicts that the current list of antimicrobial peptide is far from complete. A remarkable feature of these substances is both their broad structural diversity and wide spectrum of activity. Strikingly, the insect immune system shares several basic characteristics with the mammalian acute phase response, mainly at the level of the control of gene expression. Molecular and genetic studies with the powerful model system Drosophila have shown that the coordinate control of expression of several immune genes relies on a cascade of regulatory gene products and cis-elements that have counterparts in mammalian innate immunity. The recent characterization of immunosuppressed Drosophila mutants provides new tools for the genetic dissection of this ancestral immune system and it is anticipated that the study of insect immunity will significantly contribute to our understanding of the innate immune response in general.