Fluorescence is a highly specific signal of vegetation function, stress and vitality. Solar induced fluorescence, a very weak signal, is detectable using the Fraunhofer lines of the solar spectrum, allowing the observation from a satellite. A space mission is proposed that would provide new data type in the field of Earth observation, addressing big scale screening of terrestrial vegetation in relation to agricultural, forestry and global change issues, such as solar irradiance, ozone depletion, water availability, air temperature, pollution in air/water/soils,..). The scientific, technical and general characteristics of the proposed mission, a demonstration/validation of the technique, are exposed. The scientific payload, from a LEO sun-synchronous orbit, will measure the fluorescence in the H(alpha) (656.3 nm) and at least one other Fraunhofer line in the blue-UV region with a CCD matrix type imaging spectrometer. A thermal infrared imaging radiometer, optional although highly desirable, would measure the vegetation temperature. An additional CCD camera will provide cloud detection and scene identification. The spatial resolution will be better than 0.5 X 0.5 km2, with a nominal FOV of 8.4 degrees; a steering mirror will allow plus or minus 4 degrees across track depointing, while allowing, if necessary, freezing the image to increase the S/R. The FLEX mission will provide florescence intensities and reflectances at the same wavelengths, plus temperature if available. Processing of the data will require atmospheric characteristics (aerosols) from other missions. Interpretation of the fluorescence signal will require reflectance data over the 400 to 800 nm region; biome characteristics (LAI, architecture, biomass density factors, APAR, etc.) from space mission providing high spectral resolution, directional reflectance measurements in the visible domain; ground data on environmental factors and plant physiology and in situ florescence measurements for satellite signal validation.