Most analyses of the crisis have focused on the roles played by US mortgage lending and financial sector leverage. But our analysis shows that this view misses a large part of the picture. Enabled by the globalization of banking and a period of unusually low interest rates and risk spreads, debt grew rapidly after 2000 in most mature economies. By 2008, several countries—including the United Kingdom, Spain, South Korea, and France—had higher levels of debt as a percentage of GDP than the United States (Exhibit 1). But this crude metric is insufficient for judging whether current levels of leverage are sustainable.