Key PointsQuestionHow have emergency department (ED) capacity and use changed in California since 2011, and has the supply of acute care resources kept up with the demand for ED care?FindingsIn this cohort study of ED data from more than 400 general acute care hospitals in California between 2011 and 2021, the number of EDs decreased by 3.8% and the number of hospital beds decreased by 2.5%; the number of treatment stations increased by 21.1%, but these stations were concentrated in a smaller number of EDs. From 2011 to 2019, the total number of ED visits increased by 23.4%, and visits rated as severe with threat (Current Procedural Terminologycode 99285) increased by 67.8% over the entire study period.MeaningThese findings suggest that, although some mixed results were present, ED capacity has not proportionally expanded with the increasing California population and demand for emergency services, offering 1 potential explanation for increases in ED crowding.