This paper reviews the concept of fiscal space, or the capacity to deploy fiscal stimulus should it be needed; identifies the key factors that determine its size; and discusses considerations relevant to its use. The paper is motivated by the remarkably rapid mobilization of fiscal stimulus in the 2008-2009 global financial crisis, coordinated by the International Monetary Fund (IMF), which was followed by the equally remarkable rapid adoption of austerity in key advanced economies. This switch from stimulus to austerity occurred despite the languid pace of the global recovery, in which growth was, as subsequently described by IMF Managing Director Christine Lagarde, “too low, for too long.” At the time, the change was justified in terms of concerns over large debt burdens and diminished capacity for future action. Curiously, however, the principled voices defending the interests of future generations were silent with respect to more recent tax cuts and the adoption of higher budgeted spending.