In humans the three main groups of antimicrobial peptides are the defensins, the cathelicidins and the histatins. They differ widely in their biochemical properties and in the spectrum of their antimicrobial activities. For quite a while they were regarded only as new-type antimicrobial agents. Recent studies revealed, however, that functions of these peptides extend far beyond their antimicrobial activities. They were shown to be implicated in a remarkably broad range of other - likewise host defence related - biological processes. They proved to be important components of the innate immune system. Furthermore it was also shown that they interact with various receptors on the immature dendritic cells and lymphocytes resulting in the activation of adaptive immune responses, in which they were shown to play further immunomodulatory roles, too. Pertaining LL-37 it has even been proposed that it has more potent immunomodulatory activities than antimicrobial function. Human alpha-defensins were shown to be active across species in mice and to have immunoadjuvant effects. Recently numerous papers have been reported on studies providing abundant evidences that several diseases in humans are characterized by impairment in the function of these small host defensive peptides. The recognition of the multifunctional role of these peptides further raised the interest of the pharmaceutical industry toward them.