两栖动物，两栖类，是一个古老的品系，直接从泥盆纪晚期的叶鳍鱼进化而来，距今约3.65亿年。 它一度包含三个亚类：迷宫动物、软骨动物和两栖动物。 今天，只有两栖动物仍然存在，所有幸存的两栖动物都是其成员：蝾螈和大鲵（称为Caudata或Urodela目）、青蛙和蟾蜍（Anura目），以及很少见的无肢尾蜥（Gymnophiona或Apoda目）。

在现存的两栖动物群体中，最成功的是青蛙和蟾蜍（5453种），其次是蝾螈和蝾螈（560种），以及尾兽（171种）。 人们认为所有这些两栖动物都有一个共同的祖先，尽管一些工作者认为蝾螈和蝾螈是由一个与其他不同的古老祖先的后代。 两栖动物的物种数量与哺乳动物的物种数量大致相当，都有大约6000种，但显然哺乳动物的数量和成功率更高。

The common ancestor of lissamphibia probably lived about 290 million years ago. One fossil from that time, Gerobatrachus, is referred to as a frogamander because of its clear combination of frog and salamander-like traits. Called a "stubby-tailed salamander with froglike ears," the rediscovery of this fossil in 2004 showed that the common ancestor of frogs and salamanders lived more recently than was otherwise thought. The common ancestor of caecilians and the other amphibians may have been 10-20 million years older. The discovery of Gerobatrachus proved that modern amphibians descended from an ancient group called temnospondyls. Controversy about amphibian descent had been considered one of the most significant in vertebrate evolution prior to the discovery of the frogamander fossil.

Although ancient amphibians were relatively large, occupying many of the niches taken by reptiles and mammals today, today's reptiles are relatively small. One species reminiscent of this long-lost amphibian past is the Chinese Giant Salamander (Andrias davidianus), a river-dweller that reaches sizes of 64 kg (140 lb) and 1.83 m (6 ft). Like many other amphibians, the Chinese Giant Salamander is critically endangered, today only living in the most remote mountain streams. Other species of amphibian that are currently under danger of extinction are the Malagasy rainbow frog, Chile Darwin's frog, ghost frogs, the Seychelles frog, the Sagalla caecilian, Mexican Lungless salamanders, and the Betic Midwife Toad.