English Upper Jurassic marine reptiles of the superfamily Plesiosauroidea are reviewed in depth. Four genera and six species are recognized and described from previously known material, these being Cryptoclidus eurymerus (Phillips) C. richardsoni (Lydekker) Muraenosaurus leedsii (Seeley), Muraenosaurus beloclis (Seeley). Tricleidus seeleyi Andrews and Colymbosaurus trochanterius (Owen). In addition a skull from the Kimmeridge Clay of Dorset is described as Kimmerosaurus langhami gen. et sp. nov. The genus Cryptoclidus is described first and is taken subsequently as a model for comparison with other genera. A restoration of the skeleton of C. eurymerus is offered, and the skulls of Cryptoclidus, Tricelidus and Kimmerosaurus are reconstructed for the first time. A further three generic and twenty--five specific names are listed, discussed and rejected: some are junior synonyms but most are nomina dubia. Published work on the phylogeny and classification of the order Plesiosauria is reviewed critically. Differing family-level classifications have been produced through the arbitrary weighting of differing characters of the postcranial skeleton. Thirty-eight plesiosaur characters used by taxonomists are discussed in the light of the present work and of these fifteen (seven being skull characters) are of importance in distinguishing phyletic lineages. The evolution and phylogeny of the Plesiosauria is discussed three main lineages are identified and a new classification is proposed The order is divided into two superfamilies and four families. The Pliosauroidea represent one line of evolution and contain the single family Pliosauridae. The Plesiosauroidea contain an ancestral family (Plesiosauridae) and two descendant families (Cryptoclididae and Elasmosauridae) Diagnoses of these groups are given and referred genera listed. The Cryptoclididae comprise the genera Cryptoclidus and Kimmerosaurus together with the South American Upper Cretaceous genus Aristonectes cabrera, hitherto considered an aberrant pliosaur. The genera Muraenosaurus, Tricleidus and Colymbosaurus are primitive elasmosaurs, as also is the English Lower Jurassic genus Microcleidus Watson