This article looks at various aspects of fiscal consolidation in 18 OECD economies. The prospects for fiscal consolidation depend upon the problems a country may face with its debt stock, the political will to deal with these problems and on the costs of consolidation. These costs are a function of the impacts of fiscal policy on the economy, which is the focus of this study. The analysis is based on a series of simulations using the National Institute Global Econometric Model, NiGEM. Fiscal multipliers differ across countries because the structure and behaviour of economies differ. They also differ within countries, depending on factors such as the fiscal instrument implemented, the policy response to fiscal innovations, and expectation formation by economic agents. The purpose of this study is to allow an assessment of the likely impact on the economy and on the fiscal position of consolidation programmes.We decompose the key factors that determine the size of the multiplier by changing them one at a time. Even under a specified set of assumptions, the outturn for the budget balance retains a high degree of uncertainty. We illustrate this uncertainty by calibrating probability bounds around projected debt profiles. This can allow an assessment of the probability of achieving specified fiscal targets, such as those set out in the European Union’s new Fiscal Compact.