We compare household debt to income ratios across countries, with particular focus on the differences between the Nordic countries. We find systematic differences in levels of debt relative to income across countries over time, but a close correlation between debt cycles across countries, independently of debt levels. There are no signs of catch-up or stable equilibrium levels with respect to debt. Developments in housing markets are normally closely linked to debt growth, but provide little explanation as to differences across countries. Financial reforms also influence debt growth, but occur at around the same time in many countries. The most robust explanatory factors for the relative debt level seem to be linked to household balance sheets. Factors such as the size of household financial assets, relative levels of GDP per capita and other measures of welfare all seem to be closely correlated with differences in debt to income ratios in the cross-country comparison. This may indicate that the differences in debt are to a great extent attributable to scaling effects on household balance sheets, i.e. when households perceive their future as secure and financially sound, their debt exposure increases. Although the level of household financial assets is low in Norway, the high level of household debt is matched by large holdings of public assets.