The earliest amphibians are found in the fossil record during the Carboniferous and Permian periods, of the Paleozoic Era from 350 to 252 million years ago (mya). Over time, amphibians shrank in size and decreased in diversity, with two of the subclasses (Temnospondyli and Leprospondyli) becoming extinct, leaving only the modern subclass Lissamphibia.

The three modern orders of amphibians are Anura (the frogs and toads), Urodela (the salamanders), and Apoda (the caecilians). The number of known amphibian species is approximately 7,300, of which nearly ninety percent are frogs.

The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (Paedophryne amauensis) with a length of just 7.7 mm (0.30 in). The largest living amphibian is the 1.8 m (5 ft 11 in) Chinese giant salamander (Andrias davidianus), but this is dwarfed by the extinct 9 m (30 ft) Prionosuchus of the middle Permian period from Brazil.

The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology.

Amphibians are all tetrapods (four limbs) vertebrates that inhabit ecosystems which include: aboreal (tree-climbers); aquatic (freshwater); fossorial (underground); or terrestrial (land).

Amphibians typically start out as larvae living in water with the young generally undergoing metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. Unlike birds, mammals, and reptiles which are amniotes (having an embryonic sac), amphibians require water bodies in which to breed (anamniotes).

With their complex reproductive needs and permeable skins, amphibians are often ecological indicators and in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.