Unmanned aerial vehicles (UAVs) have gained interest in recent years in many different fields. UAVs are used for remote sensing, surveillance, precision agriculture and goods delivery, to name a few. The growing interest in their use leads to safety concerns which can be faced using a systematic approach to define safe operations. The approach proposed in this thesis consists in the definition of risk maps that quantify the risk connected with a flight operation over a certain area. Risk maps can be used for risk-informed decision making: by National Aviation Authorities to evaluate the risk of a specific flight mission, or to compute safe routes using a risk-aware path planning algorithm.