Although knowledge of the existence of cryptic species dates back hundreds of years, the true extent of cryptic diversity was not discernible until the development of modern molecular techniques. Both homoplasy and morphological conservatism—patterns which result in cryptic taxa—are rampant within the Plethodontidae, the largest salamander family and a taxon rich in cryptic lineages. The first cryptic species of plethodontid, Plethodon websteri, was described in 1979, and the discovery of new cryptic lineages has now become commonplace. Their taxonomic recognition, however, has been controversial, not a surprising result given the differences in methodology—or in the idea of what defines a species—that exist among researchers. Two radiations pinpoint the variation in forces that may drive speciation and the development of cryptic lineages. The Plethodon glutinosus complex represents a non-adaptive radiation characterized by largely parapatric, niche-conserved species. The Desmognathus quadramaculatus complex, on the other hand, is comprised of lineages that have repeatedly evolved the same homoplastic body form that is strongly correlated with niche specialization. Although many cryptic plethodontids exist parapatrically or allopatrically, several occur sympatrically, partitioning niche space more finely than previously imagined. Investigations into the biology of cryptic species should be fruitful in answering questions in both ecology and evolutionary biology and have ramifications for the understanding and conservation of biodiversity.