Estimates of groundwater recharge and discharge rates are needed at many different scales and for many different purposes. These include such diverse tasks as evaluating landslide risks, managing groundwater resources, locating nuclear waste repositories, and estimating global budgets of water and greenhouse gases. A report summarizing some of the major issues in recharge and discharge, titled Groundwater Fluxes across Interfaces, has just been released by the Committee on Hydrologic Science (COHS) of the National Research Council. The report focuses on spatial and temporal variability of recharge and discharge, their scaling relationships in space and time, and interactions of groundwater with climate. Atmospheric, surface, and subsurface parts of the hydrologic system are dynamically linked, but have very different time and space scales. Study of interchanges with the subsurface is further complicated by the diffuse nature and large extent of many recharge and discharge areas, and by the interdisciplinary character of the problem. To help address this need, the COHS convened a workshop in Egg Harbor, Wisconsin, in May 2002. The participants were asked to assess existing and potential methods of estimating recharge and discharge rates; in particular, those that hold the most promise for addressing the topics described above. Written materials provided by participants and discussions at the workshop provided much of the content of the report.