In view of the continuous increase in the number of described species of nonmethanogenic hyperthermophilic and related thermophilic archaea thriving in geothermally heated habitats (i.e., members of the orders Thermoproteales, Desulfurococcales, Sulfolobales, Thermoplasmatales, Thermococcales and Archaeoglobales), their current taxonomic status and taxonomic properties are reviewed. The introduction of 16S rRNA phylogeny has brought a more reliable taxonomic framework for these archaea, although, their phenotypic properties still remain as salient and descriptive features. Moreover, certain cellular components are potentially applicable to their taxonomic classification.