Host defence, cationic antimicrobial peptides are now recognised as an important component, in most species, of the early innate and induced defences against invading microbes. They are small (12 to 35 amino acids), cationic due to the presence of an excess of arginine and lysine over acidic amino acids, and able to fold into a variety of different secondary structures. They have highly desirable properties, such as the ability to kill rapidly a broad spectrum of microorganisms including drug resistant bacteria and often fungi at around the minimal inhibitory concentration, a low level of resistance development in vitro, the ability to protect animals against both topical and systemic infections and the capability to neutralise endotoxin and demonstrated synergy with conventional antibiotics. In addition, given the 20 building blocks (amino acids) for these peptides. even a small peptide offers enormous diversity and potential for design of improved variants. For this reason such peptides have entered clinical trials, largely as agents for topical therapy of polymicrobial infections and are considered to have excellent potential for being a novel antibiotic class.