Contoured bathymetric charts of the Celebes Sea-Sulu Sea region form the primary basis for a geologic interpretation of the region. The large ridges and basins show abundant evidence of active tectonism in the form of scarps, recent troughs, warping, large vertical displacements, and volcanoes. Terraces below the 1000-fathom depth in the basins of both seas are an enigma. Turbidity-current deposition has flattened the floors of the basins, while Pleistocene lowering of sea level caused the cutting of wide marine shelves on the ridges. Deep channels were cut in the shelves by intensive marine currents. Deep erosional features are evidence of local and regional subsidence. Published evidence regarding tectonism and geologic history combined with the marine data reveal that: (1) the left-lateral strike-slip Philippine fault extends south of the Talaud Islands and may have at least 110 km of horizontal displacement; (2) no land existed prior to Cretaceous time; (3) much spilitic lava was extruded in late Cretaceous and early Tertiary time; (4) a major orogeny occurred in late Miocene time and that another, still active, began in late Pliocene and Quaternary time.