Vegetation influences the whole ecosystem and climate. Its research and monitoring has therefore always been one of the most important fields of remote sensing. To extract some useful information from the remotely sensed data, it is necessary to understand how the measured signal is formed and what affects it. Vegetation radiative transfer models embrace this knowledge base. Ground truth data is needed for developing and validating the models. UAVSpec-series spectrometer systems have been designed for measuring the top-of-canopy forest reflectance. These are fully autonomous spectrometer systems suitable for measurements from unmanned aerial vehicles. Dark signal temperature dependences and instrument functions of the spectrometer modules were parametrized. In order to guarantee the metrological quality of spectroscopic measurements, these parameters must be taken into account in the data processing workflow. This book sheds some light to airborne spectral measurements and use of miniature spectrometer modules.