Although congressional investigations have provided some of the most dramatic moments in American political history, they have often been dismissed as mere political theater. But these investigations are far more than grandstanding. This book shows that congressional investigations are a powerful tool for members of Congress to counter presidential aggrandizement. By shining a light on alleged executive wrongdoing, investigations can exert significant pressure on the president and materially affect policy outcomes. This book constructs the most comprehensive overview of congressional investigative oversight to date, analyzing nearly 13,000 days of hearings, spanning more than a century, from 1898 through 2014. The book examines the forces driving investigative power over time and across chambers, and identifies how hearings might influence the president’s strategic calculations through the erosion of the president’s public approval rating, and uncover the pathways through which investigations have shaped public policy. Put simply, by bringing significant political pressure to bear on the president, investigations often afford Congress a blunt, but effective check on presidential power—without the need to worry about veto threats or other hurdles such as Senate filibusters. In an era of intense partisan polarization and institutional dysfunction, the book delves into the dynamics of congressional investigations and how Congress leverages this tool to counterbalance presidential power.