A methodology for parameter retrieval from multi-temporal Earth observation data is proposed and applied to forest biomass retrieval from C-band SAR backscatter. The potential of single observation C-band SAR data to map forest biomass is limited mainly due to a low sensitivity and the dependence of C-band scattering on many other parameters. In this contribution a methodology to retrieve forest biomass from multi-temporal C-band SAR observations is presented. The method was applied to map forest biomass over a large area in Central Siberia using significant stacks of ENVISAT ASAR Wide-Swath data. The resulting forest biomass map is assessed by comparison with available inventory data. The quality achieved appears very promising.