In watershed hydrology, the response of a watershed is driven by precipitation that falls on the watershed and evapotranspiration from the watershed. The precipitation may be observed as rainfall from a historical event, it may be a frequency-based hypothetical rainfall event, or it may be an event that represents the upper limit of precipitation that is possible at a given location at a specific time. Historical precipitation data are useful for calibration and verification of model parameters, for real-time forecasting, and for evaluating the performance of proposed designs or regulations. Data from the second and third categories—commonly referred to as hypothetical or design storms—are useful if performance must be tested with events that are outside the range of observations or if the risk of flooding must be described. This chapter describes methods of specifying and analyzing historical or hypothetical-storm precipitation.