OECD countries face daunting fiscal challenges following the substantial surge in debt-GDP ratios during the past four years, from already high levels in many cases. Fiscal consolidation is now the order of the day, and it takes on greater urgency against the backdrop of imminent budgetary pressures from population ageing. While strong growth would help, the bulk of consolidation will require specific structural reforms to spending and revenue programmes to stabilise and then reduce debt-GDP ratios. On the spending side, many reform options offer budgetary savings through improved efficiency, without loss of desired outcomes or adverse equity impacts. Areas examined in this paper include health care, education, infrastructure, general public services, and transfer programmes. On the revenue side, countries’ tax systems are perforated by tax expenditures that cause inefficiencies, reduce revenue, and undermine fairness. Reducing the scope and scale of tax expenditures remains one of the most promising means of boosting revenues while improving economic performance. Shifting taxation toward less inefficient tax bases also holds much promise, including raising the importance of both property taxation and environmental levies. Even without quantifying all possible measures, the cumulative cuts in spending and increases in taxation could yield 6% of GDP on average across countries in consolidation, with somewhat more on the spending side.