This chapter provides a detailed overview of the known polycationic peptides, with emphasis on those of less than 100 amino acids and with a net charge greater than +2. Cationic peptides can be classified into several groups on the basis of sequence similarities, secondary and tertiary structure, function and origin. These peptides possess antimicrobial activity against many species, including bacteria (Gram-positive and -negative), fungi, and viruses and, in the case of the more potent peptides, can have a lytic activity on mammalian cells. The oxygen-independent microbicidal host defence mechanism of mammals involves several proteinaceous molecules that are cationic in nature. These include lysozyme, bactericidal/permeability increasing factor (BPI), cathepsin G, CAP-37, lactoferrin, defensins and the eosinophil-derived proteins: the major basic protein (MBP) and the eosinophil cationic protein (ECP). Members of the defensin family possess a secondary structure rich in P-pleated sheet, which is stabilized by these intramolecular disulphide bonds. Defensins kill a wide variety of bacteria, fungi, spirochaetes, and viruses. They exert not only microbicidal activity because of permeabilization of biological membranes but they also possess chemotactic and endocrine regulatory activities. A range of inducible antimicrobial cationic peptides has been isolated—including attacins, cecropins, coleoptericin, diptericins, drosocin, phormicins, sarcotoxins, sapecins, and insect defensins. One of the main groups of antibacterial components in the insect humoral response is the cecropins. These molecules constitute a class separate and differ from other bacteriolytic peptides produced by insects by not lysing mammalian cells. The chapter discusses occurrence of cationic peptides in nature, structure-function relationships of cationic bactericidal peptides, and interactions with lipids and membranes with these peptides.