Antimicrobial peptides (AMPs) are endogenous, gene-encoded antibiotics found in virtually every life form, and a vast variety of peptides has been described. In the microbial world, AMPs help to control competitors in ecological niches and function as signal molecules in cell-to-cell communication. Among eukaryotes (plants, invertebrates and vertebrates including humans), AMPs play a key role in innate immunity on mucosal surfaces and in phagocytes. In addition to their direct microbicidal activity, for which membranes of microbes appear to be a major target, recent studies have disclosed receptor-mediated effector functions for many mammalian AMPs. Members of the defensin and cathelicidin families are chemotactic for monocytes, dendritic and T-cells and participate in shaping the adaptive immune response. The 4th GRC on Antimicrobial Peptides will address these issues from a multidisciplinary point of view, presenting the most recent advances in these areas. Moreover, given the threat of increased antibiotic resistance, ample discussion will be given to the potential of AMPs as innovative anti-infective agents for the control of plant, animal and human pathogens, and as agents for potentiating immune responses.