A large grid of deep-imaging, marine seismic reflection data has been acquired in the Gulf of Guinea. The data show that the architecture of old Atlantic igneous oceanic crust and upper mantle is highly variable, particularly if reflection Moho is taken to be the base of the crust. Most abrupt changes in oceanic basement thickness and depth to Moho can be correlated with fracture- zone crossings, but significant variations can occur between fracture zones and along flow lines, especially near the ocean-continent transition. Reflection Moho is usually continuous from ocean to continent and does not display any systematic changes in character, continuity, or reflection time even beneath the innermost shelf areas. There are several varieties of intra-crustal reflectors, including those that mark different levels within the oceanic gabbroic complex and events that diagonally link the top of oceanic seismic layer 2 and Moho. Different types of sub-Moho dipping reflections also are observed. Some are associated with fracture zones, some originate within continental crust and dip toward the ocean, dissecting Moho without offsetting it, and still others originate at the oceanic Moho and dip toward the continent. The transition from oceanic to continental crust is generally quite sharp north of lat 1°S, but the exact nature of the transition ranges from rift-block geology to abrupt juxtapositions of oceanic and continental crustal rocks. South of about lat 1°S, the transition to continental crust is more gradual, involving a progressive thickening of oceanic crust toward land. This difference may relate to the occurrence of much more oblique initial rifting north of 1°S.