The identification of the fungus Beauveria bassiana as the causative agent of white muscardine disease in silkworms and the experimental work on the transmission of this infectious agent from infected to healthy insects helped scientists like Agostino Bassi (1773–1856) and Louis Pasteur (1822–1895) to overthrow the spontaneous generation theory and champion the germ theory, which later extrapolated to human diseases. These initial reports, together with earlier observations regarding the occurrence of natural insect epizootics, assisted to devise the idea of microbial pest control and the belief that causative agents of insect diseases could be used for pest control. Today, fungus- (and oomycete-) based pesticides account over 15% of the biopesticide market. Unlike many disease-causing agents (e.g., bacteria, viruses), fungi and oomycetes are effective at colonising insects because of their cuticle-degrading armoury and their ability to evade and suppress the insect innate immune system. However, insect pathogenic fungi and oomycetes differ in their diversity and host range. Insect pathogenic oomycetes show reduced diversity (i.e. a handful of species distributed into eight genera) and a narrower host range (i.e. aquatic insects, mostly mosquito larvae). In contrast, entomopathogenic fungi can be found in six of the nine fungal phyla and infect most insect orders. This chapter represents a broad taxonomic review of the known fungi and oomycetes associated with insects and the diseases they cause.