Abbreviated introduction: The North American genus Flethodon presently has more recognised forms than any other genus of salamanders. It is the type genus of the lungless family Plethodontidae, reviewed so ably by Dunn in 1926. At present, this is the most successful salamander family; over half of the living species of the Order Caudata are plethodontids. North America is the center of the distribution of the family, but one genus, Hydromantes, also occurs in Europe, and members of several genera enter the Neotropical region. Salamanders of this family occupy habitats ranging from strictly aquatic cavernicoles (lyphlomolge and haideotriton) and mountain stream dwellers (Leurognathus) to others like Plethodon, which are completely terrestrial, even to the ex-bent of laying their eggs on land. Dunn believed that Plethodon is the most primitive genus in the attached-tongue branch of the family, and that Hemidactyiium, Ensatina, Batrachoseps, and Aneides are more specialised derivtives of a Plethodonlike ancestor. In 1926, only eleven forms of Plethodon were known to Dunn. In 1943 Bishop listed 17 species (one has since been removed from the genus) and two subspecies (both are now considered different species). In 1944, Grobman reviewed the distribution and relationships of the eastern section of the genus, including some sixteen species and subspecies. There are now twenty-five forms recognized in eastern North America, with seven more in the western United States and Canada. Grobman (1944: 266) divided the eastern forms into two groups, the Large Plethodons and the Small Plethodons. He suggested that these groups might actually represent distinct genera or subgenera, but he reserved judgement in this matter until the relationship of the western species with the eastern forms could be determined. One of the purposes of the present study is to determine these relationships by a comparative study of the morphology of all the species of the genus Plethodon. Of the twenty-five genera in the family Plethodontidae, only two others show as great or greater disjunctions in their distributions as that found in Plethodon, These are hydromantes, with two forms in Europe and three species in California, and Aneides, with four species in western North America and one in the Appalachian Mountains of the eastern United States. In the past two decades, several new plethodons have been discovered in both eastern and western North America. A review of the relationships of these forms offers a valuable opportunity to study the evolution of this important group of North American salamanders. The importance of examining plethodons in life has been emphasized by most recent workers. Many species, particularly the Large Eastern forms, are extremely difficult to identify after years of preservation. In few other vertebrates are differential characters between species so rare. For this reason, a special attempt was made to obtain living specimens of each form. Twenty-five of the thirty-two forms recognized herein have been studied alive.