The unity or disunity of the Mysidacea as competing hypotheses is currently among the central controversies in phylogenetic research on malacostracan Crustacea. The Lophogastrida, Stygiomysida, and Mysida have a great number of characters in common, stimulating decade-long discussions on the extent to which these characters are plesiomorphic within the Malacostraca and whether these taxa are to be kept separately or united in the traditional order Mysidacea. This chapter gives complementary information about the difficult birth, the rise, and the potential decline or renaissance of the order Mysidacea Boas, 1883. As typical for Malacostraca, the Lophogastrida, Stygiomysida, and Mysida share the 'caridoid facies'. Unlike Isopoda and Amphipoda, and certain taxa within the Decapoda, the pleon is never reduced. Rather, it is well developed and capable of emergency tail flipping in the Lophogastrida, Stygiomysida, and Mysida. The trunk ends in a dorsoventrally flattened telson, ventrally bearing the anus in sub-basal position. Keywords: caridoid facies; Decapoda; emergency tail flipping; Lophogastrida; malacostracan Crustacea; Mysida; Mysidacea; pleon; Stygiomysida