More than 80 years after its passage, the Smoot‐Hawley Tariff Act of 1930 still resonates in today’s debate over trade policy. Advocates of trade blame the law for deepening the Great Depression and warn of the economic damage from a reversion to protectionism. Skeptics of trade say its impact has been exaggerated. Economist and historian Douglas Irwin tells the messy and, at times, amusing story of how Congress dramatically raised tariffs in 1930 just as the world was plunging into depression, and analyzes the economic consequences of the most infamous trade bill ever enacted by Congress. Irwin then draws important lessons that can help today’s trade policymakers avoid the costly mistakes of the past.