The Mafia Deep Offshore Basin (MDOB) of southern Tanzania is one of the several East African basins which resulted from the break-up of the Gondwana continent from the Middle Jurassic onwards (Figs. 1, 2). The basin occupies an area of some 75,000 km developed between the Tanzanian continental shelf edge and the Davie Fracture Zone 200 km east of the coast. The water depth ranges form approximately 500m to a maximum of 3300m in the southeast of the basin. The availability of a modern high resolution seismic data has allowed the geological development of the basin to be interpreted in far greater detail than previously. Prior to this study the geology of the basin had been interpreted only from very widely spaced reconnaissance seismic data of the Sea Gap Group proprietary survey (1976) and the Lamont-Doherty East Africa Margin study (Coffin & Rabinowitz, 1988). The MDOB offers some of the few remaining frontier exploration opportunities in Africa. It is situated adjacent to the petroliferous Tanzanian Coastal Basin, where there are known onshore oil seeps and subsurface gas discoveries. the extrapolation of well data from this coastal area in combination with the stratigraphic and structural interpretation of the MDOB has facilitated the definition of its petroleum potential. Following licencing of the areas and subsequent exploration drilling by Petrobas, BG and Statoil, sizeable gas discoveries have been made mainly in the cretaceous sections. Further exploration is underway.