Publisher Summary The pathogens that cause disease in insects fall into four main groups: viruses, bacteria, fungi, and protozoa. This chapter discusses the primary biological properties of each of these pathogen groups, with specific emphasis on how these pathogens have been used to benefit humans. Pathogens are viruses or microorganisms that cause disease. Like all other organisms, insects are susceptible to a variety of diseases caused by pathogens. Many of these pathogens cause diseases that are acute and fatal and therefore are used as models to study processes of infection and pathogenesis as well as to control populations of insects that are pests or vectors of plant and animal diseases. Generally, insect pathogens have a relatively narrow host range and thus are considered to be more environmentally friendly than synthetic chemical insecticides. The use of the nuclear polyhedrosis viruses (NPV) of the European spruce sawfly, Gilpinia hercyniae, as a classical biological control agent. In addition to the use of NPVs in insect control, one baculovirus, the AcMNPV, has been developed as an expression vector for producing a large number of foreign proteins in vitro. This expression system takes advantage of the strong polyhedrin promoter system, which in the wild-type viruses produces large amounts of the polyhedria used to occlude virions. By substituting foreign genes for the polyhedrin gene, it is possible to synthesize in insect cell cultures large quantities of foreign proteins, such as the capsid proteins of viruses that attack the vertebrates used for vaccine development and basic biomedical research.