Pathogens causing a number of human and animal diseases use chitin and chitinases in their life cycles. Most of these diseases are caused by protozoan or metazoan pathogenic parasites. Some of these parasites contain chitin coats that protect them from the harsh conditions in the animal body or the environment. Some pathogens use chitinase to invade or exploit the chitin-containing structures of their host to establish successful infection or to be transmitted from one vertebrate to another via insect vectors. Recent studies indicate that each of these organisms has evolved to use chitin and chitinases differently and in a developmental stage-specific manner. Genes of many of these pathogenic parasites have been isolated, and the predicted amino acid sequences show a great deal of diversity. In this chapter we will discuss the roles chitin and chitinases play in several animal diseases, the strategies used to clone the chitinase genes from various parasites and the usefulness of chitinases as preventive or therapeutic agents.