The U.S. spends billions of dollars to mitigate the effects of drought. However, population growth and agricultural irrigation in water-poor regions continue to drive unsustainable use of groundwater and other fresh waters. Weather modification technologies (e.g., cloud seeding) may have the potential to increase freshwater reserves and lessen the effects of drought. Yet despite a long history of investigation and operation, only recently have advances in computer models and sensor technologies (e.g. radar) enabled the study of weather with more than simple statistical analysis. Key Questions: 1. What are the current or emerging technologies for weather modification (including materials or processes used) and what are the most recent developments in those technologies in the United States and in other countries? 2. What are the potential benefits of weather modification for the United States? 3. What challenges (including health or environmental effects) could hinder or result from the development and application of weather modification technologies? 4. To what extent are policy options warranted and what are the options?