The vehicle theft rate in the United States declined 80 percent between 1990 and 2020. Remarkably, this remains unexplained. This study examines historical evidence including reports to Congress plus automobile industry data from the Federal Register. Legislation incentivised security improvements that were fitted to high-risk vehicles from the late 1980s. Analysis of the industry data finds that theft of vehicles with electronic engine immobilizers declined 80 percent relative to a matched control group and theft of new secure vehicles declined before older vehicles. Theft declined gradually over the years as secure vehicles permeated the national vehicle fleet, the prolonged decline reflecting the arms race between manufacturers’ responses and offender effort to circumvent security. The study concludes that the electronic engine immobilizer caused the great American car crime decline. If this induced declining crime more generally, the electronic engine immobilizer may be the most important crime prevention device of recent history.