The Mastodonsauroidea, previously and more widely known as the Capitosauroidea, ranks among the most common and best known Mesozoic terrmospondyls. Mastodonsauroids exhibit wide morphological variation and significant homoplasy and their taxonomy and relationships have been in a state of flux. Since the first description of a mastodonsauroid there have been 56 genera and 118 species proposed. A comprehensive review of all these taxa has reduced the content of the group to 14 genera and 44 species. Most species previously included in the Mastodonsauroidea are undiagnostic at the generic level and designated incertae sedis within the Mastodonsauridae, Heylerosauridae, Mastodonsauroidea or Stereospondyli. Where applicable, definitions, diagnoses and a list of synonyms of all fully diagnosable taxa within the Mastodonsauroidea are given. A computer-based phylogenetic analysis of mastodonsauroid temnospondyls based on 47 cranial characters and 25 terminal taxa was performed. The analysis included the 14 genera of mastodonsauroids as well as selected well-known members of the Ly-dekkerinidae and Trematosauridae. The results supported a monophyletic Mastodonsauroidea with Benthosuchus as the sister group to a clade consisting of the Heylerosauridae and the Mastodonsauridae. The name Mastodonsauridae replaces the name ‘Capitosauridae’, because Mastodonsaurus was found to fall within the clade that includes most of the traditional ‘capitosaurid’ genera. The composition of the Mastodonsauroidea was shown to be similar to that traditionally assumed for the ‘Capitosauroidea’, and to exclude the Rhinesuchidae, a group often placed within the ‘Capitosauroidea’. The Mastodonsauroidea must have a ghost lineage extending back to at least the Late Permian, when the common ancestor of the Mastodonsauroidea and the Trematosauroidea diverged from rhinesuchid ancestors. We can therefore expect to find Late Permian representatives of the Mastodonsauroidea, perhaps present as inconspicuous elements of non-stereospondyl dominated Late Permian faunas.