Within the last decade, several antimicrobial peptides (AMPs) have been discovered. Cathelicidins are one family of AMPs characterized by a conserved cathelin domain and a variable C-terminal cationic antimicrobial domain. These peptides are produced by different cells, including leukocytes, epithelial cells and keratinocytes. Besides their direct antimicrobial function, cathelicidins can also regulate the course of inflammation and influence the mechanisms of innate immunity. In this review we discuss the biology of animal cathelicidins, their structure, expression and function.