We study the effects of the largest corporate income tax cut in U.S. history on firms and workers. To identify causal effects, we use employer-employee matched tax records and event studies comparing similarly sized firms in the same industry that faced divergent tax changes due to their pre-existing legal status. Tax cuts cause increases in firms' investment, sales, profits, employment, and payrolls, with earnings gains concentrated among highly paid workers. In the short-run, 87% of private income gains flow to the top 10% of the income distribution.