The worsening fiscal positions in both developed and developing countries brought the issues of debt and fiscal sustainability to the center of public policy debate. This paper introduces an accounting framework to study the evolution of public debt. Subsequently, this framework is applied to analyze the main determinants of the debt-to-GDP ratio of Belgium for the period 1995-2014. The framework underscores the importance of the debt composition, the exchange rate and the fiscal stance for debt dynamics, all impacting fiscal vulnerability. The results show that: (i) Belgium succeeded in quite drastically reducing its public debt in the run-up to joining the eurozone, and was able to continue doing so up until the Global Financial Crisis (GFC) hit in 2007-2008. This was chiefly accomplished by running large primary surpluses. Moreover, it was aided by a benign economic environment, i.e. relatively high real GDP growth; (ii) the bail-out of the financial sector in 2008 caused public debt to soar. Subsequently, anemic growth in combination with the cost of servicing an already high stock of debt, resulted in a steadily rising debt-to-GDP ratio. Primary deficits only played a minor role in the increase of debt, especially given the severity of the GFC.