The geophysical validation by independent means provides fundamental end-to-end information on the overall data quality. A comprehensive validation program is an inherent component of Operational Earth Observation missions. The program includes both, validation using ground-based measurements as well as inter-comparisons with other spaceborne missions. They give information on instrument characterization and calibration as well as on the performance of its processing algorithms and thus final geophysical products. Ground observations enable estimation of product accuracies, and development of regional atmospheric models and independent time series, as well as generic algorithms. The mission inter-comparisons are important to derive instrument on-orbit characterization not available from ground point collocations, such as the consistency of long time series spanning several independent and possibly not overlapping missions which are needed for climate research. ESA is committed to performing the validation of its products, including atmospheric chemistry and physics data. Detailed results for ENVISAT’s GOMOS, MIPAS, SCIAMACHY and MERIS instruments will be presented. Several targeted activities are supported that cover the validation by ground based instruments, lidars, balloons, Radio Occultation and other satellite instrumentation, as well as the inter-calibration of lidar systems in the CEOS context. Long term strategies are being developed that shall allow the validation of independent missions and are covering types of geophysical measurements rather than individual instruments or missions. In this context, CEOS/WGCV is proposing a community derived process that establishes an international quality assurance framework to facilitate harmonisation and interoperability of EO data, QA4EO.