Chitinases are produced by a wide variety of pathogenic and parasitic microbes and invertebrates during their attack on chitin-containing organisms. Examples discussed include enzymes of insect and algal viruses, of yeast killer toxin plasmids, of bacterial and fungal pathogens of fungi and insects, and of parasitic protozoa. These chitinases play roles in penetration of fungal cell walls, and of exoskeletons and peritrophic membranes of arthropods. Salivas of some invertebrate predators have chitinolytic activity which may be involved in their attack on their prey. Chitinases play a major defensive role in all plants against attack by fungi, and perhaps also against attack by insect pests. The plant chitinases form a very large and diverse assemblage of enzymes from two families of glycosyl hydrolases. At least some vertebrates, including fish and humans, also may utilise chitinases in their defence against pathogenic fungi and some parasites.