Flood frequency studies relate the magnitude of discharge, stage, or volume to the probability of occurrence or exceedance. The resulting flood-frequency functions support evaluating flood risk management projects, sizing and designing water-control measures, establishing reservoir operation criteria, and floodplain management. This chapter describes how HEC-HMS is used to develop flood-frequency functions and illustrates the approach with a case study of the Chicken Ranch Slough and Strong Ranch Slough (CRS/SRS) watershed in Sacramento County, California.