The objective of the Validation deliverable (D2.8) is to present the validation of the Sentinel-1 (S-1) Surface Soil Moisture (SSM) retrieval algorithms described in the ATBD (D2.1&D2.1A) and implemented in the Prototype Software (D2.7). The Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV) (https://lpvs.gsfc.nasa.gov/) has established standards that are currently being used for satellite SSM product validation (e.g. SMAP, SMOS, ESA CCI Cal/Val Plan). The methodologies adopted to estimate the uncertainty of the SSM products include the direct comparison of retrieved SSM values with independent measurements from in situ networks or satellite products. In this study, both approaches have been implemented and hereafter labelled as validation at local and regional scales (see the Validation Plan, D1.3).