The weather on planet Earth is a vital and sometimes fatal force in human affairs. Efforts to control or reduce the harmful impacts of weather go back far in time. In this, the latest National Academies assessment of weather modification, the committee was asked to assess the ability of current and proposed weather modification capabilities to provide beneficial impacts on water resource management and weather hazard mitigation. It examines new technologies, reviews advances in numerical modeling on the cloud and mesoscale, and considers how improvements in computer capabilities might be applied to weather modification. Critical Issues in Weather Modification Research examines the status of the science underlying weather modification in the United States. It calls for a coordinated national research program to answer fundamental questions about basic atmospheric processes and to address other issues that are impeding progress in weather modification.