Gene-encoded peptide antibiotics are ubiquitous components of host defenses in mammals, birds, amphibia, insects, and plants. Their de novo synthesis or release from storage sites can be induced rapidly, which makes them particularly important in the initial phases of resistance to microbial invasion. The endogenous antimicrobial peptides of animals are products of single genes and are synthesized as preproproteins. Multistep processing yields the mature peptide, which generally acts by inducing microbial membrane permeabilization. Several families of antimicrobial peptides have been identified that differ with respect to the presence of disulfide linkages, amino acid composition, structural conformation, and spectrum of activity. The arginine-rich three disulfide-containing beta-sheet defensins are remarkably abundant and widely distributed in animals and plants. The antibiotic peptides of higher eukaryotes merit further study for their role in natural immunity and their potential as novel therapeutic compounds.