The Kultsjön basin in northern Sweden has been the subject of detailed studies of the potential of airborne γ-ray spectrometry for snow mapping and hydrological forecasting since the spring of 1980. A brief introduction to the theory behind this technique is given. Results from five melt seasons are presented, and problems and uncertainties are discussed. A verification against ground “truth” based on snow courses, runoff measurements, and hydrological models is given. Finally it is shown how the data can be used for updating and improving more conventional forecasting models.