Abstract Mysida, ubiquitous in the fresh, brackish, and marine waters of the world, is an order with approximately 1,152 species, whereas Stygiomysida, cave-dwelling and geographically restricted, contains only 16 species. Five genes have proven useful to sort relationships within some genera and subfamilies. Strides have taken place in elucidating the relationships within the possum shrimp, but it has been a long, and still ongoing, analysis. Stygiomysida, however, appear more closely related to mictaceans than they are to the rest of the mysidans. Mysida and Stygiomysida exhibit the classic “caridoid” features. The caridoid facies, however, constitute for the most part primitive, or plesiomorphic, characters—not unique apomorphies.