This report summarizes the activities of the Water Science and Technology Board during 1991. The WSTB is intended to be a dynamic forum, a mechanism by which the broad community of water science, technology, and policy professionals can help assure high-quality national water programs. The principal products of WSTB studies are written reports which cover a wide range of water resources issues of current national concern. A few recent examples are: Restoration of aquatic ecosystems - science, technologies and public policy; Water transfers in the West - efficiency, equity and the environment; Opportunities in the hydrologic sciences; and Ground water models - scientific and regulatory applications. Projects completed, ongoing studies and published reports are described in detail in their respective sections of this report.