A close evolutionary relationship between Metazoa (animals) and Fungi was proposed over 20 years ago. The name Opisthokonta reflects the presence of a single, posteriorly directed flagellum found on the reproductive cells of most metazoans and some fungi. Subsequent molecular work confirmed this relationship and also identified several protistan groups (including the choanoflagellates, ichthyosporeans and nucleariids) belonging to this clade. In this chapter we review the literature on this group in order to describe the opisthokont lineages and the current thinking about how they are related to each other. The phylogeny of the opisthokonts is far from complete and we will discuss the areas that need to be addressed, as well as current evidence on the possible sister-groups of the opisthokonts.