Certain problems associated with a drought frequency analysis are exposed and their solutions are proposed. The techniques are based on a probabilistic framework. Return period of a project's design drought, the annual drought for which a water resource project such as a treatment plant or a low flow augmentation system has to be designed, is treated as a function of the project's useful life and the desired level of ‘assurance.’ (‘Assurance’ here stands for the probability of nonoccurrence of droughts equal to or smaller than the design drought during the project's useful life.) A method is shown for estimating the probability and return period associated with the design drought of a project. The paper presents a chart that gives recurrence intervals of design droughts for various combinations of the estimated useful life of a project and the desired level of assurance.