This chapter discusses morphological and taxonomic clarification of the genus Plesiosaurus. Detailed examination of the broad range of English Liassic plesiosaur species indicates that only Plesiosaurus dolichodeirus and its junior synonyms can be assigned to Plesiosaurus. Many named Plesiosaurus species are based on inadequate material and considered nomina dubia. While others even those contemporaneous with P. dolichodeirus, are certain or probable representatives of distinct genera exhibiting significant cranial autapomorphies. The chapter elaborates that only the German Lower Jurassic, notably the Holzmaden Lagerstatte, and the French upper Toarcian currently contain taxa that are potentially valid specific variants of Plesiosaurus. P. dolichodeirus has relatively short antorbital and temporal regions, subcircular temporal fenestrae, a broad but pointed rostrum, and a large pineal opening. P. dolichodeirus also has a nonspatulate mandibular symphysis, approximately 40 cervical vertebrae with distinctly doubled rib facets, slightly rugose articular edges to virtually all vertebrae, a sharp posterolateral expansion to the coracoid, slightly longer forelimbs than hindlimbs, a curved humerus with a shallow interepipodial groove on its ventral surface, but no sharp anterodistal comer, and other identifying features.