Principal aquifers in California are the main underground water sources found within the state's borders. According to the Oxford Dictionary of Environment and Conservation, an aquifer is a layer of rock or soil that allows water to flow through it and is covered by a layer of rock that does not allow water to pass through.

California has identified 515 groundwater basins and subbasins within these aquifers. These basins may be managed by local water districts. For example, the Coachella Valley Water District manages the groundwater in California's Coachella Valley groundwater basin (CA groundwater basin no. 7–021). This basin is located in the Colorado River hydrologic region, one of 13 top-level water regions in California. These state water regions are similar but not exactly the same as the federal hydrologic unit system's surface-water drainage basins. The California Department of Water Resources provides detailed descriptions of each of the 515 recognized groundwater basins online, including in PDF format.

In the United States, principal aquifers are organized using codes and names assigned by the National Water Information System (NWIS) of the United States Geological Survey. Each aquifer is identified by a three-digit code based on the age of the rock layer and a four- or five-character abbreviation for the name of the geologic unit or aquifer.