This paper provides the first quantification of the contribution of mergers and acquisitions (M&A) to rising industry concentration. We use comprehensive firm-level microdata from the U.S. Economic Census to show that M&A account for 44% of the increase in aggregate concentration. We classify M&A using the numerical criteria in the merger guidelines and show that presumed anticompetitive M&A account for 12% of the increase in aggregate concentration. We provide evidence indicating that low rates of intervention by the federal competition agencies reflect limited regulatory capacity rather than successful deterrence that prevented firms from attempting anticompetitive mergers.