Carmen Reinhart and Kenneth Rogoff tell the history of financial crisis as a tale of excessive public debt. But what more commonly drives financial instability, says Moritz Schularick, is excessive private debt. Financial crises are credit booms gone bust. Schularick and his collaborators compile a long-run data set of disaggregated credit flows, separating loans for productive investment from loans for the purchase of existing assets. A marriage of economic history and modern statistical methods to investigate the role of finance in the macroeconomy — this is new economic thinking.