Pythium is an important genus of Oomycetes, consisting of hundreds of species, of which many are plant pathogens. Pythium species survive well in moist soils and are widespread in different tropical, subtropical and temperate regions of the world. Their sexual spores, named oospores, enable them to survive long periods of time under harsh environmental conditions. Pythium species can be transmitted via different means including soil, potting media, infected seedlings, and irrigation water. They are mainly soilborne pathogens, causing diseases in several important vegetable crops, cereals, and ornamental plants. Damping-off, root rot, wilt, fruit rot and seed decay are common and widespread disease symptoms caused by members of the genus Pythium, resulting in significant losses all over the world. Several cultural, chemical, and biological methods have been developed for the management of Pythium diseases. This chapter focuses on the taxonomy, biology, distribution, transmission, host range and control measures of Pythium species.