This paper examines how the understanding of the prospectivity of the Liberia-Sierra Leone Basin has developed over the last 40 years largely due to improvements in seismic data over this period. A series of basins developed between major transform fault zones associated with the opening of the South Atlantic Ocean from the early Cretaceous onward. The Liberia-Sierra Leone Basin forms part of the West African Transform Margin that extends from Sierra Leone to Benin. Early hydrocarbon exploration (1972–1985) was targeted at shelfal structural synrift traps of Lower Cretaceous age. None of the wells drilled in this period was regarded as a discovery. In 2001, a new regional 2D seismic survey allowed a new play, deepwater Upper Cretaceous channel fans, to be identified. More recent 3D seismic using the latest data acquisition and processing technologies, including AVO, has enabled drillable prospects to be identified. Wells drilled in 2009–2010 offshore Sierra Leone have established that a working hydrocarbon system exists in this basin.