As part of the science and research element of ESA's Living Planet Programme, the FLEX mission is currently a candidate for the 8^th Earth Explorer as an Opportunity mission. FLEX will be the first mission designed to globally map and monitor chlorophyll fluorescence emission originated from the core of the photosynthetic machinery in terrestrial vegetation. FLEX mission concept consist in a single platform with a hyperspectral instrument (FLORIS) acquiring in the spectral range between 500 nm and 800 nm. FLEX will fly in tandem with GMES' satellite Sentinel-3 (S3), exploiting the synergy between their data and helping in the characterization of the atmospheric state, essential for a reliable retrieval of fluorescence emission. Combining measurements from FLEX and S3 instruments, the mission shall provide accurate and reliable information on the overall photosynthetic activity.