The report begins with a key based on external characters and a tabulation of diagnostic cranial characters for separating Pithecia from Chiropotes and Cacajao of the subfamily Pitheciinae. The description of Pithecia is based on external, cranial, dental, and certain postcranial characters with particular attention to limb proportions and the femoral third trochanter. The species of Pithecia are arranged in the Pithecia pithecia group with P. p. pithecia and P. p. chrysocephala, and the Pithecia monachus group with P. m. monachus, P. m. milleri, P. irrorata irrorata, P. i. vanzolinii (new subspecies), P. aequatorialis (new species), and P. albicans. The discussion on nomenclature compares the usage of names by various authors for the different taxa. The species previously identified by Hershkovitz [Hershkovitz, 1979] as Pithecia monachus proves to be the new species P. aevuatorialis, whereas the sympatric species he called P. hirsut Spix (1823)s the same as true P. monachus É. Geoffroy (1812). Speciation among sakis is expressed, for the most part, by marked differentiation in coloration and head pelage patterns in males. Females of corresponding males diverged along the same lines but to such lesser degree that specific discrimination between them is complicated. Details of sexual dimorphism in coloration and pelage patterns are described, and those of size are documented by summaries of weights and by measurements of external, cranial, dental, and skeletal characters for each sex of each taxon. A key to the species and subspecies of Pithecia is followed by an account of each taxon that includes statements of taxonomic history or synonymy, type specimen, type locality, and geographic distribution. A full description with the addition of accounts of diagnostic characters, variation, comparisons, measurements, list of specimens examined, and other matters, is provided only or the new forms Pithecia aequatorialis and P. irrorata vanzolinii. Comparable descriptions of all taxa are contained in Volume 2 of Living New World Monkeys, currently in preparation. The gazetteer, however, includes complete data for all saki collecting and recorded localities plotted on the range maps.