Antimicrobial peptides, which have been isolated from many bacteria, fungi, plants, invertebrates and vertebrates, are important components of the naturel defenses of most living organisms. A wide variety of human proteins and peptides also have antimicrobial activity and play important roles in innate immunity. Cathelicidins and defensins are major groups of epidermal antimicrobial peptides. The highly active nature of antimicrobial peptides in vitro suggests these peptides might be effective in treating bacterial infections in vivo.