Yucca Mountain, located in southern Nevada in the Mojave Desert, is being considered as a geologic repository for high-level radioactive waste. Although the site is arid, previous studies indicate net infiltration rates of 5--10 mm yr-1 under current climate conditions. Unsaturated flow of water through the mountain generally is vertical and rapid through the fractures of the welded tuffs and slow through the matrix of the nonwelded tuffs. The vitric-zeolitic boundary of the nonwelded tuffs below the potential repository, where it exists, causes perching and substantial lateral flow that eventually flows through faults near the eastern edge of the potential repository and recharges the underlying groundwater system. Fast pathways are located where water flows relatively quickly through the unsaturated zone to the water table. For the bulk of the water a large part of the travel time from land surface to the potential repository horizon (~300 m below land surface) is through the interlayered, low fracture density, nonwelded tuff where flow is predominately through the matrix. The unsaturated zone at Yucca Mountain is being modeled using a three-dimensional, dual-continuum numerical model to predict the results of measurements and observations in new boreholes and excavations. The interaction between experimentalists and modelers is providing confidence in the conceptual model and the numerical model and is providing researchers with the ability to plan further testing and to evaluate the usefulness or necessity of further data collection. ¿ 2001 American Geophysical Union