Advanced technologies for making chemical, biological, and nuclear weapons can be purchased from hundreds of companies around the world. But negotiating treaties to regulate this trade would take years—and even then, many governments lack the will or resources to enforce them. Security experts often suggest that the United States could avoid these difficulties by encouraging industry to govern itself. Recent experience in the artificial DNA industry shows that this approach can yield large dividends. At the same time, officials need practical guidance about when private standards are possible and what government can do to promote them. The authors argue that private security standards work particularly well for industries in which manufacturers face massive fixed-cost investments, sell to large buyers, and face risk from intelligent adversaries. This profile fits most high-technology industries that security experts care about. The authors also discuss strategies that governments can use to promote, influence, and learn from private initiatives to regulate dangerous technologies.