The goals of the Global Energy and Water Cycle Experiment Continental-Scale International Project (GCIP) point to the need for high-resolution data sets on all elements of the land surface and atmospheric hydrologic cycle. A high-resolution precipitation data set has been derived from radar reflectivity observations taken from the National Weather Service WSR-88D radars in the continental U.S. The data set is available for a continuous five-year period (1996–2000) at an hourly, 4 × 4 km2 resolution for the Mississippi River Basin. Development of the data set involved data management and quality control of input radar-reflectivity, parameter estimation for radar-reflectivity transformation, and product accumulation and quality control of the precipitation product. Quality control algorithms for the input radar-reflectivity included procedures to deal with radar calibration differences, an especially important problem in developing a long-term, continental-scale data set for diverse hydroclimatological applications. Rainfall estimation was based on a Z-R conversion algorithm that involved an optimization technique to determine the parameters for the transformation of radar-reflectivity to rainfall. Rainfall accumulation involved integrating to hourly, 4 × 4 km2 resolution and then visually inspecting the final product. Some limitations of the algorithm are presented and suggestions are proposed for improving the development of a long-term, large-scale precipitation product. Initial comparisons of the radar-based product with a rain gauge based product after a quality analysis of both products show good agreement in the Mississippi River Basin.