The City of San Diego is taking a regional leadership role to protect its drinking water supplies. The City draws source water from nine local reservoirs, located within five different catchments in San Diego County. The watershed lands tributary to the reservoirs spread across the City of San Diego, San Diego County, several small cities, tribal lands, and federal lands, covering over 900 square miles of diverse landscapes. Nearly all of the watershed lands draining into the raw water reservoirs are located outside City boundaries, beyond the City's jurisdiction and authority to control land use planning, zoning, and building codes. Although the City of San Diego Water Department (SDWD) evaluates and comments on new developments in its watersheds, it has little direct control over most new developments which are occurring outside City boundaries. To date, the City has attempted to influence watershed development on an "ad hoc" basis, addressing each project case-by-case, with limited success. Thus far the watershed lands have remained largely rural and undeveloped. However, as the San Diego region grows the future holds the promise of extensive residential and commercial development in the watershed lands and the potential for water quality impacts. These factors highlight the need for greater focus on source water protection. To provide a framework for stronger protection of water quality in its drinking water reservoirs, the SDWD has recently embarked on a project to establish "Source Water Protection Guidelines for New Development". The Guidelines will establish water quality control measures for urban runoff from new development within the watersheds and specific to each of the reservoirs. In developing the Guidelines, the SDWD is working closely with the other agencies in the area that have jurisdiction over planning and land use to build on existing planning, zoning, and building code regulations and practices and to develop a uniform framework that is workable within the context of existing local and regional programs. Representatives from many of these agencies have taken an active role in the development of the Guidelines, serving on a Technical Advisory Committee that meets regularly to review project milestones and provide input. This paper focuses on the critical first step toward developing the Source Water Protection Guidelines - establishing a credible scientific basis and need for source protection in the SDWD watersheds. Includes reference, tables, figures.