During the economic and financial crisis, fiscal positions across the OECD countries deteriorated sharply. This raises the question of what level of primary deficit would ensure long-term sustainability and what degree of consolidation is needed. The purpose of this paper is to gauge the scale of fiscal consolidation that will be needed to ensure long-term sustainability. The analysis uses so-called fiscal gaps to provide a simple metric for how much consolidation is needed under a series of different assumptions and scenarios. The aim is to highlight the scale of the problems, how they differ across countries and the uncertainties surrounding the estimates. A first set of results suggest that lower debt targets provide greater room for manoeuvre to react to shocks in the future. A second set of results shows that growth-enhancing structural reforms �| especially reforms of pension systems �| can mitigate budget pressures resulting from ageing populations and hence contribute to fiscal consolidation. Furthermore, raising efficiency in the provision of health care and education can reduce budgetary pressures. Finally, achieving debt objectives under shocks to interest rates or to government spending would require additional tightening in most of the OECD countries.