The Los Angeles Coastal Plain (LACP) covers about 580 square miles and is the largest coastal plain of semiarid southern California. The LACP is heavily developed with mostly residential, commercial, and industrial land uses that rely heavily on groundwater for water supply. In 2010, the LACP was home to about 14 percent of California’s population and is also a major commercial and industrial hub. There has been a heavy reliance on groundwater from the LACP for many years. Managed aquifer recharge from the spreading grounds and barrier wells is now a substantial component of the LACP’s groundwater supply. The U.S. Geological Survey (USGS) conducted a study in cooperation with the Water Replenishment District of Southern California (WRD) assessing the historical and present status of groundwater resources in the LACP and the development of tools to better understand the groundwater system. The cornerstone of these tools is the development and application of the Los Angeles Coastal Plain Groundwater-flow Model (LACPGM-USG) to simulate past groundwater conditions, estimate groundwater-budget components and flow paths, and approximate future groundwater conditions under different scenarios (https://doi.org/10.3133/sir20215088).