This chapter discusses the biology of the Phoronida. Larval development and the systematics of the Actinotroch larvae are also presented. The main anatomical features of a Phoronida are presented diagrammatically. Phoronid species are either hermaphrodite or dioecious. The transport of the sperm to the female or hermaphrodite species is affected by means of the spermatophore. The main mechanism of insemination is the penetration of the sperm mass into the metacoelom through the nephridial duct; this is the natural access to the ovary. Fertilization in Phoronida appears to be internal. Cross-fertilization is a rule in hermaphroditic species. Fertilization occurs in the trunk coelom usually just after the egg escapes from the ovary. The characteristic Phoronid larva is termed Actinotrocha. The general form and the gross structure of the Actinotrocha are familiar and are briefly described to facilitate the understanding of the different larval stages and the processes of metamorphosis. The feeding characteristics of Phoronida are also reviewed.