This chapter describes the taxonomy of Appendicularians (larvaceans), small free-swimming pelagic zooplankton. They belong to the phylum Chordata, which includes the vertebrates. Like other tunicates Appendicularia are relatively primitive forms compared to the vertebrates, although they exhibit three features that are common to all chordates at some stage in their life: gills slits, a tubular nerve cord, and a primitive spinal column, the notochord. The chapter covers their life cycle, ecology, and general morphology. It includes a section that indicates the systematic placement of the taxon described within the tree of life, and lists the key marine representative illustrated in the chapter (usually to genus or family level). This section also provides information on the taxonomic authorities responsible for the classification adopted, recent changes which might have occurred, and lists relevant taxonomic sources.