The present chapter introduces concepts and methods related to risk and risk assessment of water shortages due to droughts in water supply systems. The main aim of the chapter is to provide methodologies able to quantify in a probabilistic way the risk of failure. Two procedures for unconditional (planning) and conditional (operation) drought risk assessment are proposed. Both are based on Montecarlo simulation of the water supply system, in order to better take into account the stochastic nature of the hydrological input to the system. The proposed methodologies results in an effective aid during both the planning and operating stages giving valuable information about expected frequency and amount of deficits related to the demands supplied by the system under study.