Microsporidiology, the study of microsporidia, is regarded as a long-standing scientific discipline that dates back to the nineteenth century, when a microsporidian infection outbreak stunned the whole European silkworm industry. The other microsporidian species mainly in fishes and insects came into existence later. But this fungus-like organism engaged the researcher’s inquiry for many years. Most microsporidian species are important pathogens of insects, but recent studies have shown that these also cause common diseases in crustaceans, fishes, and even human beings; probably having been transmitted through contaminated water or food. This fact has thus made microsporidia a favored subject of investigation among biologists studying parasitism. This chapter summarizes broad research regarding microsporidia and marks their role in biology.