The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission will be launched in 2007. Its goal is the global and frequent measurement of soil moisture over the land and surface salinity over the sea, two key parameters governing the complex global climate. SMOS’ single payload is the Microwave Imaging Radiometer by Aperture Synthesis (MIRAS), the first space-borne interferometric radiometer. SMOS will provide brightness temperature data over a wide range of incidence angles at vertical and horizontal polarizations (dual-polarimetric mode) or the full Stokes emission vector (full-polarimetric mode), from which the geophysical parameters will be derived. This paper focuses on the soil moisture retrieval problem using dual or full-polarimetric information. In this case, the brightness temperatures, as measured by the radiometer, depend mainly on five parameters descriptive of the surface under study: vegetation opacity and albedo, and soil surface temperature, roughness and moisture. Some of these parameters can be derived from other sensors or can be inferred from the multi-angular brightness temperatures themselves. Simulation results using the SMOS End-to-end Performance Simulator (SEPS) developed at the Universitat Politecnica de Catalunya (UPC) will be presented and discussed.