Cryptobranchids are the largest extant salamanders in the world, with the largest species, Andrias davidianus, exceeding 5 feet in length. There are two genera, Andrias, with two Asian species, and Cryptobranchus, the only extant American species. All species are fully aquatic, paedomorphic, and increasingly rare in the wild. The three species are very similar in appearance, with minute eyes, paddle-shaped tails, and large folds of highly vascularized skin along the sides of the body that increase oxygen absorption. Adults lack gills, but retain gill slits (open in Cryptobranchus, closed in Andrias), and do not possess eyelids. The limbs are short and powerful, and help keep the salamanders positioned on the floors of their stream habitats. Females may produce up to 500 eggs in a season, which are paired in strings. Fertilization is external, and the male guards the eggs until hatching. Larvae possess short gills to reduce drag in flowing waters, streamlined bodies, and impressive tails for propulsion. 

Andrias japonicus is found in central Japan, Andrias davidianus in central China, and Cryptobranchus alleganiensis in eastern United States. Cryptobranchids are found in shallow to medium depth, cold, flowing waters. They are found under large rocks and other debris, and blend in nicely to the brown bottoms of rivers and streams. Cyrptobranchids possess very sensitive skin that help them detect and peruse prey in the wild. They are capable of detecting light rays when shined on the body. 

Fossil Cryptobranchids were larger than the modern, reaching more than 2 meters in length. North American Cryptobranchid fossils are known as far back as the Paleocene, Asian fossils date as far back as the Middle Jurassic, and European fossils can be traced to the Pleistocene (Mousetrap, 2003). The latest discovery is a fossil Cryptobranchid, Chunerpeton tianyiensis, discovered in volcanic deposits in the Jiulongshan Formation (Bathonian), Inner Mongolia, China, in 2003 (Gianaro, 2003). This extinct member has been dated to the Middle Jurassic Period, some 165 million years ago. This is one of the oldest known fossil representation of salamanders. This fossil serves as evidence that the Cryptobranchids separated from the Hynobiids in Asia, before the Middle Jurassic. Recently an even older caudate fossil has been discovered in Europe; more details on this find can be found in the Salamandridae section below.

Fossil remains of Andrias scheuchzeri (Scheuchzer, 1726) were found in Germany, and dated to the Miocene Epoch, Tertiary Period. A. scheuchzeri was originally thought to be human remains from Noah's flood, and was given the name Homo diluvii testis, meaning "head of a flood man". This fossil became known as the "Diluvian Man", and was thought to be that of a man that may have witnessed the universal flood described in the biblical tale of Noah's Ark. In the early 19th century, Georges Cuvier discovered that the fossil was actually the remains of a giant salamander. The genus name Andrias was later adopted by Tschudi, meaning "Man-Image"; a reflection of its initial description.

Small to medium sized salamanders found mostly in Asia, with the exception of Salamandrella keyserlingii (extending into Russia), and Batracuperus persicus (found in the Middle East). All species have external fertilization, although Ranodon sibiricus produces spermatophores. The family Hynobiidae is related to the family Cryptobranchidae, with which it forms the Suborder Cryptobranchoidea (primitive salamanders). Keratinization is present on the fingers and toes in some species, the so-called Clawed Salamanders of the genus Onychodactylus. Some species have well-developed lungs, while others possess reduced lungs, and the genus Onychodactylus lack lungs altogether. 

Most species breed in waters after a dormant period spent on land, with the exception of Hynobius kimurae, who spends the dormant period underwater. During the aquatic phase, males of many species develop remarkably swollen heads, and laterally compressed tails. Females deposit crescent-shaped egg sacs in streams or ponds, where they are fertilized by the male. Larvae are either stream or pond type, depending on the species, and possess the typical external gills and caudal fin specialized for their environment.

Previously, the family Hynobiidae was divided into two subfamilies, Hynobiinae, and Ranodontinae. The genera Pachypalaminus, Batrachuperus, Hynobius, and Onychodactylus comprised Hynobiinae, while Ranodontinae consisted only of Ranodon. Today, the genera Pachypalaminus, Paradactylodon, and Xenobius, are synonymized with Hynobius, Batrachuperus, and Pachyhynobius, respectively (Obst, Jacob, Richter, 1989). 

Ambystomatidae is comprised of one extant genus, Ambystoma, and approximately 30 species. Most species complete metamorphosis, and are terrestrial and fossorial, however, some are permanent neotenes. Ambystoma mexicanum, the Mexican Axolotl, is perhaps the most recognizable obligate neotene, and is often a specimen of research and development. A. mexicanum is thought to be extinct in the wild, with the only remaining populations existing in captivity. Fortunately, A. mexicanum has been regularly bred for centuries, and is not on the brink of extinction, despite its absence from the wild. 

Ambystomids are restricted to North America, and can be found from Alaska to Mexico. The terrestrial species are typically robust salamanders, with thick limbs and tails. Ambystoma mavortium and Ambystoma tigrinum are considered the longest terrestrial species in North America, rivaled only by the terrestrial Dicamptodontids, and surpassed only by the aquatic Cryptobranchus alleganiensis (Hellbender). A. mavortium,  A. tigrinum, and other Tiger Salamanders also have large larvae, reaching up to 5 inches before metamorphosis. Superficially, these large larvae resemble adult and young A. mexicanum, but will metamorphose under normal conditions. Because of their similar appearance, A. mexicanum and larval tiger salamanders are often confused in captivity. To further add to the confusion, the name waterdog is often applied to both species, while a true waterdog is a member of the family Proteidae.

Unisexual hybrids occur throughout the Great Lakes region to New England and the Maritime Provinces of Canada (Petranka, 122). Hybrids may be diploid, triploid, polyploid, or infrequently, tetraploid, or pentaploid. Unisexual hybrids are female, and can consist of varying combinations of genetic material from A. laterale, A. jeffersonianum, A. texanun, and A. tigrinum (Bogart, 1989; Bogard and Licht 1986; Kraus 1985). Some unisexual biotypes have been given names, A. nothagenes, A. platineum, and A. tremblayi, although these names are considered incorrect today.

Ambystomids are members of the suborder Salamandroidea, the advanced salamanders. The families Dicamptodontidae and Rhyacotritonidae have both been included in Ambystomatidae, but were removed based on genetic information. Dicamptodontidae is a sister taxa to Ambystomatidae, while Rhyactotritonidae is more distant. Current studies suggest that the Ambystomatidae-Dicamptodontida clade is sister to Salamandridae. The sharing of a similar spine pattern also had Ambystoma included in Plethodontidae at one point (Mousetrap, 2003). 

The extinct genus Amphitriton is known from the upper Pliocene, and fossils of the extant genus Ambystoma are known from the lower Oligocene through the Pleistocene, in North America (Mousetrap, 2003). 

Amphiumidae is comprised of one extant genus, Amphiuma, and three species, all of which are found in the southeastern United States. Amphiuma are large, elongate, nearly-limbless salamanders, often described as eel-like in appearance, and sometimes reaching more than a meter in length (except the one-toed amphiuma, which reaches only around 40cm). The three species can be differentiated by the number of toes on each limb, one, two, and three, and by their size as adults. Like many other caudates, amphiuma are paedomorphic, retaining gill slits into adulthood. Amphiuma are also the only caudate species that possess internal gills. The limbs of all three species are reduced, and serve limited purpose in locomotion in adults. In larvae, however, the limbs are much larger compared to the body size, and can be used for walking. Amphiuma also possess lungs, and lack eyelids and tongues. 

Amphiuma are aquatic, inhabiting densely planted ponds and swamps. They are capable of exiting the water to forage for foods, and may do so on especially wet nights. Amphiuma are also capable of enclosing their bodies in a water-tight mucus, allowing them to survive in a state of suspended animation for months, sometimes more than a year, if their water source were to dry up. Eggs are laid on muddy land areas, near water, and are guarded by the females. Larvae make their way to the water during rainy nights.

Amphiumidae is part of the suborder Salamandroidea, the advanced salamanders, all of which are internal fertilizers. Amphiumidae was originally placed in its own suborder Amphiumoidea, but this model has been rejected based recent clade analyses.

There are three North American  fossil species known from the upper Cretaceous through the upper Miocene. The extinct genus Proamphiuma is known from the Cretaceous. Those from the Pleistocene were found in the southeastern United States (Mousetrap, 2003).

Dicamptodontidae is comprised of four species, in the single genus Dicamptodon. All species are found in western North America, from California to southern Canada, and the Rocky Mountains of Montana and Idaho. Dicamptodons are collectively referred to as Pacific Giant Salamanders, a reflection of their large, robust physique, and western distribution. In general, Dicamptodons resemble Ambystomids, with thick limbs and tail, and large head, but are larger than most Ambystomids. Dicamptodon copei is normally paedomorphic, retaining gills and aquatic behavior into adulthood, however, metamorphosis is possible.

Dicamptodons inhabit coniferous forests, and cold streams or creeks during the breeding season (Petranka, 1998). The lifecycle is biphasic and the larval stage can last up to 5 years. Unique to D. ensatus is the ability to vocalize when disturbed.

Dicamptodons are members of the suborder Salamandroide, the advanced salamander. The genus Dicamptodon was formerly included in Ambystomatidae, and removed based on genetic information. The families Dicamptodontidae and Ambystomatidae are considered sister taxa. Current studies suggest that the Ambystomatidae-Dicamptodontidae clade is sister to Salamandridae (Mousetrap, 2003).

Fossils of two extinct genera and the extant genera are known from the Paleocene in North America. Three extinct genera are known from the Paleocene and Miocene in Europe (Mousetrap, 2003).

Plethodontidae is the largest, and most diverse of the ten caudate families. They are distributed almost exclusively throughout the Americas, with a small group of Hydromantes populating Southern Europe. Plethodontidae includes neo-tropical genera that inhabit the Southern Hemisphere in Central and South America. The family Plethodontidae consists of two subfamilies, Desmognathinae, and Plethodontinae. The latter subfamily consists of three tribes, Bolitoglossini, Hemidactyliini, and Plethodontini.

Plethodontids have adapted to an array of habitats, including arboreal, aquatic, terrestrial, and fossorial. The typical biphasic lifecycle is observed in most species of the subfamily Desmognathinae and tribe Hemidactyliini, with a few that are are strictly terrestrial with no aquatic larval stage. The tribe Hemidactyliini are also has permanently aquatic, paedomorphic or perennibranchiate species. Such species are often troglobitic (cave dwellers), possessing reduced eyes and pigment. The tribes Bolitoglossini and Plethodontini lack aquatic larvae and hatch as miniature adults from terrestrial eggs (Larson, 1996). Members of the tribe Bolitoglossini of the New World tropics are unique in having adapted to the tropical life. Some species are arboreal or fossorial, and most possess webbed feet. An arboreal lifestyle is also observed in the genus Aneides, of the tribe Plethodontini.

Members of the subfamily Desmognathinae, collectively called Dusky Salamanders, are found exclusively in North America, especially the eastern United States. Dusky salamanders are terrestrial, inhabiting temperate forests and brush areas. Depending on who is consulted, there are between 10 and 20 recognized species. Most are biphasic, with the exception of D. aeneus, D. wrighti, and Phaeognathus hubrichti, which produce terrestrial eggs that hatch fully formed young. D. marmoratus and D. quatramaculatus are aquatic and semi-aquatic, respectively, both with an unusually long larval period of up to three or four years. 

All Plethodontids are lungless and rely mainly on cutaneous and/or branchial respiration. Metamorphosed adults rely solely on oxygen absorption through their most skin, whereas neotenes and some larvae rely mainly on branchial respiration (gills). 

Plethodontidae is included in the suborder Salamandroidea, the advanced salamanders, all of which are internal fertilizers. Based on phylogenetic analyses of RNA sequences, it is concluded that the Plethodontids do not have any close relatives in existence today.

North American Plethodontid fossils are known from the Miocene through the Pleistocene (Mousetrap, 2003).

The family Proteidae is comprised of six or seven, paedomorphic, fully aquatic species, often called mudpuppies or waterdogs. All species are found in North America, except the single species in the genus Proteus, which occurs in southeastern Europe. The terms Mudpuppy and Waterdog stem from the belief that these species make barking sounds when disturbed, however, this is not true, as all species are incapable of vocalizing. 

Mudpuppies and waterdogs comprise the genus Necturus. They are noted for their bushy, red gills, and secretive behavior. Adults also retain two gill slits (Petranka, 1998). Necturus species are all similar in appearance, although varying in size, with dark mottled dorsa that blend into their watery floors. Necturus are found in ponds, lakes, and streams in eastern North America. Proteus anguinus is very different in appearance and behavior than the mudpuppies and waterdogs of North America, possessing a whitish pink, translucent body, lacking eyes, and inhabiting limestone caves of southeastern Europe. 

Proteids are members of the suborder Salamandroides, the advanced salamanders. Two fossil genera are recognized, Mioproteus from the Miocene into the Caucasus Mountains, and Orthophyia from the Miocene in Germany. Necturus fossils are known from North America dating to the Paleocene and the Pleistocene, and fossil Proteus are know from the Pleistocene in Germany (Mousetrap, 2003).    

Rhyacotritonidae is comprised of four species, in the single genus Rhyacotriton. This genus was formerly comprised of one species, Rhyacotriton olympicus. Originally, Rhyacotritonids were placed in the family Ambystomatidae, later  in Dicamptodontidae, and finally placed into their own family in 1992. Rhyacotritonids are collectively referred to as Torrent Salamanders, and are found exclusively in the western United States, specifically northern California, Oregon, and Washington. Torrent Salamanders are found in coniferous forests, and well-oxygenated streams during the breeding season. All species have the typical biphasic lifestyle, with aquatic larvae and terrestrial adults. Rhyacotritonids can be differentiated from other northwestern species by their notably square-shaped cloaca (Stebbins, 2003).

Rhyacotritonidae is included in the suborder Salamandroidea, the advanced salamanders.

Salamandridae contains perhaps the most recognizable species, the newts and "true salamanders". There are arguably 65-70 species distributed throughout North America, Europe, Russia, the Middle East, Asia, and the northern tip of Africa. Salamandridae includes the newts, which are characterized by their relatively small size, moderate to highly aquatic behavior, toxicity, biphasic lifecycle, and their diverse breeding behavior and sexual dimorphism. Included in the newt group are the genera Cynops, Neurergus, Notophthalmus, Pachytriton, Paramesotriton, Pleurodeles, Taricha, Triturus. Depending on the source, the genera Salamandrina, Echinotriton, Euproctus, and Tylototriton are sometimes included in the newt group because they produce aquatic larvae that hatch from eggs, and reproduce in water, although they lead more terrestrial lives than those species that fit the mold of a typical newt. 

Although traditionally used to describe species only from the genus Salamandra, particularly Salamandra salamandra, the first described salamander, the term "true salamander" is often applied to all species in the family Salamandridae outside of the newt group. The latter usage of the term would then include the genera Chioglossa, Mertensiella, and Salamandra in the "true salamander" group. "True salamanders" are not necessarily "more" salamander than the other species, rather these species can be thought of as the epitome of a typical salamander, in the sense that they are used as a model for what a salamander should look like.

There are many species in other caudate families that fit the image of a "true salamanders", and so this term is somewhat broad and non-descriptive. It seems fitting that the newts comprise a rather distinct group in the family Salamandridae, while the term "true salamander", when applied to all remaining members outside of the newt group, may be used simply to exemplify the distinctness of the newts, and when used in its traditional manner may be applied only to the genus Salamandra. In summary, all newts are salamanders, but not all salamanders are newts.   

The word newt has a long history, originating from the Anglo-Saxon word efete or evete, which later became ewte in Middle English. Over time, it is presumed the term an ewte was construed into a newte (Cogger & Zweifel, 2003). 

Newts are characterized by their rough skin (except Pachytriton), and lack of "slimy" mucus. Newts are also relatively small, with the largest species reaching around 10 inches in length, sometimes slightly longer. However, the majority are around 3-6 inches in length, with the smallest only reaching about an inch. Many newt species are brightly colored, or have bright colored abdomens to warn of their toxicity. The toxins present in newts was termed Tarichatoxin, after its isolation in newts of the genus Taricha, and is a diluted form of Tetrodotoxin, or TTX (for more information on newt toxicity, see article 0011 - Toxicity and Defense Methods of Amphibians). Most species are semi-aquatic, or mostly-aquatic, with some experiencing a terrestrial juvenile stage.   

Newts are found primarily in Europe and Asia, with a few species in North America, the Middle East, and northern Africa. Newts display an array of behaviors, from mostly aquatic to mostly terrestrial, and everything in between. Many species are terrestrial part of the year, and return to bodies of water to reproduce during their season, while others may remain aquatic year round. Taricha in particular are unique in that they migrate remarkable distances to the same ponds they emerged from to breed themselves. Other genera also migrate to breeding ponds, but are less picky, sometimes settling for puddles or ditches. Triturus in particular are noted for their extraordinary courtship behavior, and sexual dimorphic characteristics (in most species), which may include the production of a large dorsal crest, webbed feet, and a change of colors on certain parts of the body. Triturus species also partake in elaborate courtship "dances" where the males go to great lengths to impress the females. Some species such as those of the genera Pleurodeles, Taricha, and Notophthalmus may exclude an elaborate courtship display, and opt for an embrace, called amplexus, that may last for several hours, up to a day or more. Males of such species may develop cornified nuptial pads on the toes and inner legs that aid in gripping the female in amplexus. Other species, such as Tylototriton, Cynops, Pachytriton, and Paramesotriton, do not amplex, and may develop mild sexual dimorphic characteristics, usually in the form of laterally compressed tails, smoother skin, and bluish or whitish sheen on the body and tail. Courtship from such species usually consists of the mail fanning pheromones toward the female with the tail (tail-fanning), accompanied by alert and bold behavior, but may be as mild as circling snout-to-tail in the water. Tail-fanning is seen in may newt species, often in combination with other courtship behavior, and is not limited to the fore mentioned species. Still, others may develop unique characteristics, such as toe flaps, or a thin thread at the end of the tail. Euproctus asper is especially unique, in that it is the only species to reproduce through direct cloacal contact. Euproctus asper's breeding behavior consists of a complex form of amplexus, where the male and female look almost knotted together.

Some species and subspecies of the genera Salamandra and Lyciasalamandra, do not have the typical biphasic lifecycle. These species/subspecies viviparous, producing fully formed miniature adults, and passing the aquatic larval phase within the mother. Viviparous salamanders produce few young, usually between 1-4 each year, compared to the hundreds of eggs that may be produced by biphasic newts each year. 

Fire salamanders are often regarded as typical salamanders, in that they are of average size (around 8 inches long) and proportion, colorful, toxic, and possess four normal limbs, and a tail. Chioglossa, on the other hand, are rather elongate, with the majority of total length contributed by the tail. Chioglossa are either terrestrial, or semi-aquatic, and display ventral amplexus during the breeding season. The long, thick tail of Chioglossa stores food reserves, and can be shed in the presence of danger (autotomy). The food reserves in the tail serve an important purpose, and can negatively affect the fertility of females if dropped off, or missing for other reasons. The true salamanders portray limited sexual dimorphism, usually in the form of swollen cloaca (in males), and laterally compressed tail on those that reproduce in the water. 

All species of the family Salamandridae are internal fertilizers, a characteristic of the suborder Salamandroidea. The biphasic lifecycle is present in most species, except the viviparous species. The aquatic larvae of newts may metamorphose into terrestrial juveniles, sometimes called efts, that will return to the water several years later to reproduce, or semi-aquatic young that never enter a terrestrial phase. Facultative neoteny is present in some species, when induced by their natural habitat, or in artificial environments. Some dimorphic characteristics, such as smooth skin, laterally compressed tails, and color change, are sometimes described as "reverse metamorphosis", as such characteristics indicate mild reversion to the larval form. The species Salamandrina terdigitata is regarded as a salamander, but is probably more similar to the newt group based on morphological similarities, the biphasic lifecycle, and larval development. There is also controversy over the monophylic status of the genera Mertensiella and Triturus. Other suggested models include the monophyly of the true salamanders, and monophyly of all newts except Salamandrina. Salamandrina separated from the remaining lineages of the family Salamandridae near the time of separation from the newt ancestors.

Fossils of the family Salamandridae are widespread in Cenozoic deposits throughout Europe, from the Eocene through the Pleistocene. There is an extinct genus known from Miocene in Asia. Fossils of the extant North American species are known as far back as the Oligocene (Mousetrap, 2003). In 2003 a section of a scull was accidentally discovered on a beach in North Tynside in 1993, has been determined to have belonged to a previously unknown, giant newt-like animal. The fossil was given the name Kyrinion martilli, and dated back to the Carboniferous (possibly Devonian) era, some 300-350 million years ago. This is the oldest known fossil caudate, and surpasses the recently discovered Chunerpeton tianyiensis (Cryptobranchidae) by more than 150 million years. Kyrinion martilli was a 6-foot long, newt-like animal, thought to have been one of the earlier ancestors that gave rise to amphibians in one branch, and reptiles and mammals in another.

All species are paedomorphic, retaining their larval gills and 1-3 gill slits. The forelimbs are reduced, and serve minimal function in adults. The hind limbs are absent in all species, a feature that distinguished Sirens from any other caudate species. Sirens are capable of enclosing their bodies in a mucus that protect them from desiccation during dryer months, when they may be found aestivating in the mud. The dwarf sirens, Pseudobranchus, attain lengths up to 6 inches (39cm), whereas the lesser and greater sirens can reach up to 18 inches (46cm) and 39 inches (100cm), respectively.   

Sirenidae is the sole family in the suborder Sirenoidea. Sirens are often placed in their own suborder due to the fact that they are presumed to be external-fertilizers, but are too morphologically different from members of Cryptobranchoidea and Salamandroidea to be included in these suborders. Breeding behavior has not been formally documented for Sirens, and they are only presumed to be external fertilizers. This conclusion was reached based on the absence of cloacal glands used by internal-fertilizers. Sirens differ so much from other caudates, that they are sometimes placed in their own order, Meantes or Trachystomata, but recent analyses place them as a sister taxa to all remaining salamanders (Mousetrap, 2003; Cogger & Zweifel, 2003).