Plesiosaurs were among the first fossil reptiles discovered, the eponymous  Plesiosaurs, was named in 1821.

Plesiosaurus was one of the first of the marine reptiles to be discovered and excited great interest in Victorian England. It was so-named ("near lizard") by William Conybeare (7 June 1787 - 12 August 1857 and Henry De la Beche (10 February 1796 - 13 April 1855), to indicate that it was more like a normal reptile than Ichthyosaurs, which had been found in the same rock strata just a few years earlier.

Plesiosaurs had a broad flat body and short tail. Their limbs had evolved into four long flippers, which were powered by strong muscles attached to wide bony plates formed by the shoulder girdle and the pelvis. The flippers made a flying movement through water. Plesiosaurs breathed air, and bore live young; there are indications that they were warm-blooded.

Since the Plesiosaur discovery, the first in in the Plesiosaurian genus there has
been more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an improved understanding of their anatomy, relationships and way of life.

Plesiosaurs showed two main morphological types. Some species, with the "plesiosauromorph" build, had (sometimes extremely) long necks and small heads; these were relatively slow and caught small sea animals. Other species, some of them reaching a length of up to seventeen metres, had the "pliosauromorph" build with a short neck and a large head; these were apex predators, fast hunters of large prey. The two types are related to the traditional strict division of the Plesiosauria into two suborders, the long-necked Plesiosauroidea and the short-neck Pliosauroidea.