Through evolution, nature has provided practically all organisms with secondary metabolites: chemical compounds that provide the producing species with specific protective mechanisms or other advantages. Among these, there are, e.g., peptides that display antibiotic/antimicrobial activity. These have been isolated from plants, invertebrates, vertebrates, and humans, but also from microorganisms. Some antibiotic peptides are an essential part of innate immunity, which has evolved in most organisms to combat microbial challenge. In addition, antimicrobial peptides sometimes also display cytotoxicity against mammalian cells.