This chapter describes the taxonomy of Lophogastrida and Mysida, small shrimp-like crustaceans commonly known as ‘opossum shrimps’ because of the ventral brood pouch in the female. Because of their highly adaptive nature, they have progressively invaded new areas via new connections of water systems and through ballast water. 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.