Amphibians employ a number of different methods of respiration, with juveniles using gills and skin before they metamorphose into adults that use lungs and skin. All amphibians keep their skin at least somewhat moist at all times to enable gas exchange. Their thin skins have an array of blood vessels underneath to utilize the absorbed gases. Different species, even in the same environment, vary in their respiratory organs.
Most amphibians begin life as aquatic animals. Most amphibian gills are actually external structures, which are fully exposed, rather than the more protected gills of fish. Some salamanders never transition from having such gills, remaining aquatic animals with gills their whole lives. Conversely, a number of amphibians are born as land animals, with lungs instead of gills. Other species have neither lungs nor gills, relying entirely on their skin for gas exchange.
Amphibian lungs, when present, are much different than mammalian or other more advanced lungs. Mammalian lungs have a sponge-like structure that drastically increases their surface area and allows far more efficient gas exchange. Amphibian lungs are much simpler, sac-like organs. Because they are cold-blooded, amphibians' energy needs are generally less than those of mammals and they tend to use their lungs only occasionally.