This paper reports the results of compositional analysis of obsidian from 94 sites in the Andean region. One hundred forty-one specimens were analyzed by neutron activation (naa), while 812 specimens were analyzed by x-ray fluorescence (XRF). Both analytical techniques are described, with a rationale for certain aspects of the XRF analysis. Cultural data on obsidian use in the Andes is also provided. The bulk of the article describes the distribution in space and time of major obsidian compositional groups. Sixteen compositional groups were identified in the sample. Eight were apparently fairly important in antiquity. Only one source, the Quispisisa source, was located and analyzed. It provided obsidian for a core area including Huancayo, Huancavelica, Ayacucho, and Ica from Preceramic times until the decline of the Huari empire. Two other major spheres were also identified in the south highlands, one in the Titicaca Basin and the other in Cuzco. These two spheres showed little interaction with other parts of the Andean region, thus strengthening earlier views of Peruvian culture history, in which these areas are seen as culturally and economically isolated throughout most of prehistory. Chavin and Huari are associated with shifts in obsidian distribution. This report was destined to be published in Spanish in Revista del Museo Nacional, Lima.