Antimicrobial peptides are effector molecules of innate immunity that provide a first line of defense against pathogens. In mammals, they are stored in granules of leukocytes and are present in those sites that are exposed to microbial invasion, such as mucosal surfaces and skin. In the last decade, biochemical investigations and recombinant DNA technology have allowed the identification and characterization of several antimicrobial peptides from various animal and vegetal species. Most of the mammalian peptides have been grouped in two broad families: defensins and cathelicidin-derived peptides. Functional studies have shown that the toxicity mechanisms for many peptides consist of a rapid permeabilization of the target cell membrane. In addition to their microbicidal activity, some members of both families are multifunctional molecules, playing a modulating role in the inflammation and the antigen-driven immune response.