The Airborne Prism Experiment (APEX) instrument is a hyperspectral remote sensing imaging spectrometer developed by a Swiss-Belgian consortium on behalf of ESA. It is being operated jointly by VITO Centre for Remote Sensing (Belgium) and RSL (Switzerland). http://www.apex-esa.org/. APEX is a dispersive pushbroom imaging spectrometer covering the solar reflected wavelength range between 372 and 2540 nm with nominal 312 (max. 532) spectral bands. APEX is calibrated using a combination of laboratory, in-flight and vicarious calibration approaches. These are complemented by using a forward and inverse radiative transfer modeling approach, suitable to further validate APEX data. It is flown on board the DLR dornier aircraft during EUFAR flights.