One of the University of Chicago Law School’s best known ideas or outputs over the last fifty years is that the common law (made by judges and often passed down and adapted over many years) is efficient. It was an idea advanced by Richard Posner, with respect to tort law, in his time as a professor here, but it is also reflected in his and other judicial opinions which students across the country meet in almost every non-constitutional course. What does this idea really mean, and is it plausible or even correct? If yes, why did the common law decline in influence? Statutes and regulations have far more impact on our present-day lives than does the common law. Judges are now known and evaluated for their constitutional decisions rather than for what they do in contracts and torts and other areas that are often described as common-law subjects. Could the common law solve our current concerns about climate change and autonomous vehicles? Saul Levmore is the William B. Graham Distinguished Service Professor of Law. This Chicago's Best Ideas lecture was presented on October 15, 2018.