There are ten living amphibian families, grouped into three suborders. Hynobiidae and Cryptobranchidae comprise the primitive or ancient suborder, Cryptobranchoidea. Modern caudates, including Salamandridae, Plethodontidae, Rhyacotritonidae, Amphiumidae, Ambystomatidae, Proteidae, and Dicamptodontidae, form the advanced salamander suborder, Salamandroidea. Sirenidae are placed in their own suborder, Sirenoidea. The fossil family Batrachosauroididae is included in Salamandroidea, while the fossil family Karauridae is placed in its own suborder, Karauroidea.

As of today, there are more than 500 recognized caudate species; a number that changes often to reflect new and redefined species. The largest caudate group is the lungless salamanders, family Plethodontidae, which comprises more than half of all known caudate species. Plethodontids are found almost exclusively in the Americas, with a large radiation into the tropical Central and South Americas. A small number of plethodontid species are also found in Europe, and, surprisingly, an Asian plethodontid salamander was described in 2005. The smallest groups are the giant salamanders (Cryptobranchidae), and Amphiuma (Amphiumidae), for which there are only three living species in each family. For more information about each caudate family, see the Caudate Families and the Taxonomic Model section.

All caudates, or urodeles, possess tails, a general characteristic that separates this order from Anura (frogs). Some living caecilians, order Gymnophiona, also possess tails, but differ from salamanders in that caecilian tails are generally indistinguishable from the body, and are highly reduced compared to caudates. The tails of most caudates are obvious, being approximately of equal length to the body, however, some may possess extraordinarily long tails (Oedipina, etc.), while the tails of others may be hard to distinguish from an elongate, snake-like body (Amphiuma, etc.). Caudates also possess four limbs of relatively equal size, with the exceptions of the Sirens, which lack hind limbs. Unlike the anurans, caudates do not leap to move from one place to the next, but rather walk, sometimes run. Aquatic species may walk along the floor of their watery homes, and are rather capable swimmers, using their well-developed tails for propulsion. Some aquatic species walk well on land, while others, such as the amphiuma, possess rudimentary limbs that serve little purpose for locomotion.

Although highly variable on the surface, caudates all share a few fundamental characteristics. These characteristics define the caudate group, and include the following: (1) absence of an otic notch and middle ear, (2) presence of a large footplate and short stylus on the columella in most taxa, (3) absence of postorbital, postparietal, tabular, supratemporal, jugal, quadratojugal (present in Karauridae), supraoccipital, basioccipital and ectopterygoid bones, (4) presence of ribs, (5) presence of true teeth on both jaws, (6) gill slits and external gills in aquatic larvae (when present), (7) origin of the adductor mandibulae internus superficialis muscle on the top and back of the skull (except Karauridae) and small size of the levator mandibulae posterior (Duellman & Trueb, 1986; Larson, 1996).

Many caudates are biphasic, emerging from eggs as aquatic larvae, and later metamorphosing into terrestrial, aquatic, or semi-aquatic adults. Biphasic species typically reproduce in water, however there are some exceptions, such as Ambystoma opacum, a species that deposits eggs on land, and waits for seasonal rains to raise the water levels and submerse them. 

Some species can be considered only partially biphasic in that they do not metamorphose completely, but retain larval characteristics into adulthood, and reproduce in this semi-larval state. Such species are referred to as neotenic (also sometimes called paedomorphic). The families Amphiumidae, Sirenidae, Cryptobranchidae, and Proteidae retain larval characteristics into adulthood, to varying degrees. There are also neotenic species, and neotenic tendencies in the families Salamandridae, Ambystomatidae, Plethodontidae, Dicamptodontidae, and Hynobiidae. Perhaps the most famous neotene is the Mexican Axololt, Ambystoma mexicanum. This species is commonly used in laboratory research, and is often kept and bred in captivity. For more information about neoteny, see the Biology section, and Caudate Families and the Taxonomic Model.

A few species, including some Salamandrids, are viviparous, producing fully formed miniature adults, as opposed to eggs. In these cases, the larval stage is passed within the mother. Such species produce considerably less offspring, usually around 1-4, compared to the hundreds of eggs that may be deposited each season by some biphasic species.

Many Plethodontid salamanders deposit eggs on land, which pass the aquatic larval stage within the egg casing, before emerging as fully formed miniature adults. Such species do not require bodies of water to reproduce, but must still maintain a level of moisture to sustain life.

Adult Plethodontids are also unique in that they lack lungs, and rely mainly on cutaneous respiration and buccopharyngeal respiration. Adults of most other species utilize a combination of buccopharyngeal, cutaneous, and pulmonary respiration, and sometimes branchial (gill) respiration. Adult amphiuma lack external gills, although they are nearly completely aquatic, and rely mainly on pulmonary respiration. Many neotenes retain external gills into adulthood, and like aquatic larvae, rely heavily on branchial respiration. 

Caudates are found mainly in the cooler Northern Hemisphere, with the exception of some genera of the family Plethodontidae that inhabit tropical zones of Central and South America, and a few other unique species. North America is home to the greatest phylogenetic diversity of caudates, which includes species from 9 of the 10 living amphibian families. The only family without representatives in North America is the Asiatic family Hynobiidae. The distribution map at right shows the approximate global range of caudates. 

Larson, Allan. (1996). Caudata. Tree of Life Web Project.
http://tolweb.org/tree?group=Caudata&contgroup=Living_Amphibians. (Accessed: 2003).