Although vegetation monitoring has been one of the key objectives of many different satellite missions in the past, FLEX will be the first mission explicitly designed to monitor the photosynthetic activity of the terrestrial vegetation layer by using a completely novel technique measuring the chlorophyll fluorescence signal that originates from the core of the photosynthetic machinery. This will provide a completely new possibility to assess the dynamics of actual photosynthesis through sun-induced fluorescence, which offers a great advancement over current capabilities that can only detect potential photosynthesis as derived in passive reflectance measurements by conventional land surface monitoring satellites. While current observational techniques provide information about the amount of light absorbed by vegetation, FLEX will provide direct insight on how much of such absorbed energy is actually used for photosynthesis, this being a completely new type of information never available before from space observations.