The FLuorescence EXplorer (FLEX) mission proposes a satellite for the global monitoring of steady-state chlorophyll fluorescence in terrestrial vegetation. After more than 70 years of basic and applied research in chlorophyll fluorescence, it is now clear that fluorescence is a sensitive probe of photosynthetic function in both healthy and physiologically perturbed vegetation. Fluorescence is a powerful non-invasive tool to track the status, resilience, and recovery of photochemical processes and moreover provides important information on overall photosynthetic performance with implications for related carbon sequestration. The early responsiveness of fluorescence to atmospheric, soil and plant water balance, as well as to atmospheric chemistry and human intervention in land usage makes it an obvious biological indicator in improving our understanding of Earth system dynamics. The amenability of fluorescence to remote, even space-based observation qualifies it to join the emerging suite of space-based technologies for Earth observation. FLEX would encompass a three-instrument array for measurement of the interrelated features of fluorescence, hyperspectral reflectance, and canopy temperature. The program would involve a space and ground-truthing program of 3-years duration and would provide data formats for research and applied science.