A comprehensive compendium on the taxonomy and biology of the 3106 living and fossil species of the world's termites is presented, along with reviews of Isoptera morphology and evolution, identification keys, the history of termite systematics, and summary of the world's 363 significant pest species. A complete bibliography is provided of nearly 5000 references covering virtually all aspects of termite taxonomy and biology through December 2011. The morphology of Isoptera is thoroughly reviewed and illustrated with original scanning electron micrographs and photomicrographs, covering the cuticular anatomy and those internal organs that are taxonomically and phylogenetically significant, including several new character systems. Terminology is presented for the systems of tibial spines and spurs so as to establish homologs. Keys are presented to the nine living families of termites, and the world subfamilies and genera of Archotermopsidae, Hodotermitidae, Kalotermitidae, Mastotermitidae, Rhinotermitidae, Serritermitidae, Stolotermitidae, and Stylotermitidae. A key to subfamilies of the Termitidae is included. A detailed morphological diagnosis for each family and subfamily is provided, along with images of exemplar species. The history of isopteran research in taxonomy, systematics, morphology, paleontology, and biology is reviewed from 1758 to the present, with emphasis on transformative workers such as Holmgren, Silvestri, Emerson, Roonwal, Noirot, and Sands. Evolution of the Isoptera is reviewed, including the diversity and natural history of genera and species in all zoogeographic regions, major patterns in social biology, the phylogeny of Recent and fossil genera and families, and 135 million years of fossils preserved as compressions, mineralized replicas, and in amber. The definitive sister group to the Isoptera is the monogeneric family of wood roaches, Cryptocercidae (Cryptocercus), so the taxonomic ranks of the two groups are now Infraorder Isoptera and Infraorder Cryptocercoidea within Order Blattaria (roaches and termites). The compendium summarizes the taxonomic history, nomenclature, distribution, type locality, and repository, and all significant aspects of natural history and biology for each species of the world, exclusive of pest control and colony inquilines (termitophiles). The classification of Recent and fossil lower termites (all those exclusive of family Termitidae) used in the compendium is from Engel et al. (2009), which is based on morphology and largely congruent with molecular studies. Rhinotermitidae s.s. (exclusive of Stylotermitidae) may be paraphyletic with respect to Termitidae, although the six traditional subfamilies of the former are used here. A separate section summarizes the nomenclatural changes made in the compendium, including new synonymies, new combinations, status novus, lectotype selection etc. A detailed list is provided of museums and other institutional collections that house type specimens. An index is included. The Treatise is intended to provide an authoritative foundation for taxonomic work on the Isoptera, present and future.