Abstract This chapter introduces the Oligochaeta annelids, here treated as a paraphyletic stem group of the class Clitellata separated from the Hirudinea, Acanthobdellida, and Branchiobdellida. It starts by considering the group in a phylogenetic and taxonomic framework, as well as in terms of geographical distribution and diversity. It then deals with the general biology of these worms in describing their anatomy, physiology, reproduction and life history. Ecological and behavioral aspects of Oligochaeta are next addressed through topics related to habitat distribution and limitations due to physiological constraints, feeding behavior, predators and parasites, competition, impact on the environment, and their use as organic pollution indicators. By the end of the chapter, the scientific study of the Oligochaeta is addressed via practical aspects such as collecting, culturing, and specimen preparation for identification.