Using novel technology and dedicated algorithms, the FLuorescence EXplorer (FLEX) will yield information about the health of the world’s plants. The information gathered will be used to improve our understanding of how carbon moves between plants and the atmosphere and how photosynthesis affects the carbon and water cycles. The FLEX mission offers two high-level products for exploitation by end-users: the Level-1C (L1C) and Level-2 (L2). In this context, a comprehensive strategy for the validation of FLEX operational products is planned. This strategy ensures the traceability to the mission requirements and guarantees that all parameters relevant for the operational products have been adequately validated. The FLEX mission requirements are highly challenging and they cover the end-to-end Earth observation system including high-level requirements, mission operations, data product development and processing, data distribution and data archiving. Risks associated with the validation of the FLEX products have been identified, possible mitigation scenarios have been outlined and back-up solutions proposed. In this paper, we provide an overview of the FLEX strategy for the calibration and validation (cal/val) of the satellite’s operational Level 1C and Level 2 science data products. It also outlines instruments and measurements needed to assess spatial and temporal comparison and possible sampling strategies and tools for upscaling ground measurements.