Discrimination between self and non-self is the basic function of innate immunity that is found in both vertebrates and invertebrates. A critical first step in any immune response is the recognition of an invading organism as foreign. In the innate immune systems of both vertebrates and invertebrates, such recognition, termed pattern recognition, is mediated by a group of proteins, known as pattern recognition proteins or receptors. Different pattern recognition proteins recognize and bind to molecules (pathogen associated molecular patterns, PAMPs) present on the surface of microorganisms but absent from animals. Binding of pattern recognition proteins to PAMPs triggers responses such as phagocytosis, nodule formation, encapsulation, activation of proteinase cascades, melanization and synthesis of antimicrobial peptides. The characterizations and functions of several classes of pattern recognition receptors involved in immune responses of the invertebrate, including peptidoglycan recognition proteins, thioester-containing proteins, Gram negative binding proteins, scavenger receptors, C-type lectins, galectins, Toll-like receptors and hemolins were reviewed.