The Malacostraca appear to have been derived from epibenthic ancestors with many caridoid features. Despite their very long palaeontological record the Phyllocarida differ in so many respects from the basic eumalacostracan morphological and functional plan that they are regarded as unlikely ancestors and more probably an early diverging branch. The general morphology of the malacostracan cephalothorax and carapace is discussed and it is shown that the concept of a maxillary segment carapace common to all Crustacea is not valid. The malacostracan dorsal shield is produced by a fusion of terga and a free carapace fold is sometimes formed at its posterior margin. Some functional systems of the various caridoid Malacostraca are discussed. Attention is drawn to the unsolved problem of secondary segmentation after the alleged loss of a cephalothorax, e.g. in the Syncarida. The unsatisfactory status of the diagnosis of the superorder Peracarida is pointed out and a revision recommended.