Plethodontid salamanders have become model organisms for studying many fundamental questions in evolutionary biology, ecology, and behavior. Dozens of studies focused on geographic patterns of phenotypic and genetic variation have been conducted on plethodontids, such that we now have a deep understanding of how many species are in the family and where they occur. This body of work on patterns of speciation has been foundational to exploring the drivers of the major patterns of species richness in the family. Approximately 70% of the world's 696 extant species of salamanders are plethodontids. The distribution of plethodontid species diversity exhibits striking variation globally. Most plethodontids are concentrated into two hotspots of diversity, the Appalachian Highlands and the Mesoamerican Highlands. Moreover, within these montane centers of diversity more species are found in midelevation habitats than at the highest or lowest elevations. The clade has been highly successful in the New World tropics, which harbors more plethodontid species than the temperate zone. Here, I review new insights on the evolutionary and ecological causes of variation in plethodontid species richness over geographic space and evolutionary time. New hypotheses on the phylogenetic relationships of plethodonids and tools from comparative phylogenetics and biodiversity informatics have been critical to this recent progress. Threats to plethodontid diversity arising from global climate change are examined, as is the need to further study and to cross-validate forecasts of species' range dynamics with different modeling approaches and independent data.