A two-dimensional mathematical model considering coupling between a deforming elasto-visco-plastic fold thrust belt, fexural subsidence and diffusional surface processes is solved using the Finite Element Method to investigate how the mechanical behaviour of brittle-ductile wedges infuences the development of foreland basins. Results show that, depending mainly on the strength of the basal decollement, two end-member types of foreland basin are possible. When the basal detachment is relatively strong, the foreland basin systemis characterised by: (1)Highly asymmetrical orogen formed by thrusts concentrated in the incoming pro-wedge. (2) Sedimentation on retro-side takes place in one major foredeep basin which grows throughout orogen evolution. (3)Deposition on the pro-side occurs initially in the foredeep, and continues in the wedge-top before isolated basins are advected towards the orogen core where they become uplifted and exhumed. (4) Most pro-wedge basins show an upward progression from lowaltitude, foredeep deposits at the base to high altitude, wedge-top deposits near the surface. In contrast, when the basal detachment behaves weakly due to the presence of low viscosity material such as salt, the foreland basin system is characterised by (1) Broad, low relief orogen showing little preferential vergence and predominance of folding relative to faulting. (2) Deposition mainly in wedge-top basins showing growth strata. (3) Many basins are initiated contemporaneously but form discontinuously due to the locus of active deformation jumping back and forth between different structures. Model results successfully reproduce first order observations of deforming brittle-ductile wedges and foreland basins. Moreover, the results support and provide a framework for understanding the existence of two main end-member foreland basin types, simple and complex, associated with fold-thrust belts whose detachments are relatively strong and weak, respectively.