Antimicrobial peptides (AMPs) are part of the armament that insects have developed to fight off pathogens. Insect AMPs are typically cationic and often made of less than 100 amino acid residues. Although their structures are diverse, most of the AMPs can be assigned to a limited number of families. The most common structures are represented by peptides assuming a alpha-helical conformation in organic solutions or disulfide-stabilized beta-sheets with or without alpha-helical domains present. The diverse activity spectrum of these peptides may indicate different modes of action. Genetic analysis in the Drosophila model evidenced that multiple signal transduction pathways are activating the genes coding AMPs.