The Santa Clarita Valley is a rapidly urbanizing semi-arid valley in northern metropolitan Los Angeles, California (Figure 1). The local groundwater resources have remained a viable supply during the valley’s 40-year urban history because of management strategies conceived and implemented by the five local water purveyors. Local management activities have included (1) funding basin-scale hydrogeologic studies and monitoring programs, (2) preparing annual reports on the valley’s water supplies and demands, including the state of the local aquifers and a water supply outlook for the following year, (3) entering into cooperative working agreements with the downstream water agency (in Ventura County), and (4) preparing and adopting a formal Groundwater Management Plan. This plan specifies the operating program for the valley’s shallow and deep aquifers. A key technical activity under the plan has been the development of a detailed numerical model of the local groundwater basin. The model has been used to improve the conceptual understanding of the groundwater system and its relationship to surface water, and to evaluate the sustainability of different operational alternatives under the plan. The investment in the model also has given the purveyors a tool for evaluating more targeted questions and problems, such as the potential effects of global warming on aquifer recharge, the potential feasibility of riverbed infiltration programs in enhancing groundwater sustainability, and how best to implement mitigation activities in response to local containment events. In summary, groundwater management in the Santa Clarita Valley is conducted primarily at the local level. Because of local initiative and cooperation, this groundwater basin has retained its sustainability despite continued urbanization, and it has remained one of the few basins in southern California that has not had to pursue a legislative solution or legal remedy to groundwater resource management.