California Jam was a rock music festival co-headlined by Deep Purple and Emerson, Lake & Palmer, held at the Ontario Motor Speedway in Ontario, California, on April 6, 1974. The festival attracted 300,000 paying music fans and set what were then records for the loudest amplification system ever installed, the highest paid attendance, and highest gross in history. It was one of the last of the original wave of rock festivals, as well as one of the most well-executed and financially successful, and presaged the era of media consolidation and the corporatization of the rock music industry.