Through the work of a number of investigators, a tremendous amount of geochronological data with excellent geological control exists on the numerous ophiolitic assemblages of the Sierran-Klamath orogen. Distinct ophiolitic assemblages of latest Precambrian, Ordovician-Silurian, Carboniferous-Permian, Late Triassic–Early Jurassic and late Middle–Late Jurassic (Callovian-Oxfordian) ages are recognized. The geochronological data, in conjunction with structural and petrologic data, facilitate a relatively detailed analysis of the petrotectonic and structural-stratigraphic development of the orogen from the perspective of ophiolite basement geology. Insights into the plate tectonic settings of ophiolite genesis and emplacement are offered by an abundance of information that now exists on active oceanic island arcs and related environments. Remnants of abyssal lithosphere, boundary transform melange, extensional fore-arc igneous sequences, and interarc basin lithosphere are all preserved within the Sierran-Klamath assemblages. In general, the older assemblages or the older elements of polygenetic assemblages have greater affinities to abyssal lithosphere, whereas the younger assemblages or elements reflect suprasubduction zone environments of genesis. The upper Paleozoic through Jurassic assemblages occur in regional belts, and their tectonic, as well as petrogenetic, histories may be related in detail with some confidence to the tectonic development of the Cordilleran margin.