Understanding California’s water balance sheet—how much there is, who has claims to it, and what is actually being “spent”—is key to effectively managing the state’s limited water supply in support of a healthy economy and environment. The latest drought has spotlighted serious gaps in California’s water accounting system. California is a large, geographically diverse state, and its water systems are physically interconnected and institutionally fragmented. Water infrastructure connects the state’s northern watersheds to its southernmost communities, Sierra Nevada rivers to coastal cities, and surface water to groundwater. Additional complexity arises from having hundreds of independently governed water systems, each with its own water accounts; from the widespread practice of managing linked surface water and groundwater as separate systems; and from a lack of clarity on how much water is reserved for environmental purposes. The combination of physical interconnectedness, institutional fragmentation, and water scarcity heightens the need for more effective accounting at the statewide and river-basin levels.  We identify gaps in California’s water information systems and recommend that the state modernize its water accounting, and that key state agencies—supported by an oversight committee of key stakeholders and independent experts—develop and adopt a common accounting framework.