The impact of fiscal policy on output and its components has long been a central part of fiscal policy analysis. But, as has been made clear by the recent debate over the likely effects and desired composition of fiscal stimulus in the United States and abroad, there remains an enormous range of views over the strength of fiscal policy's macroeconomic effects, the channels through which these effects are transmitted, and the variations in these effects and channels with respect to economic conditions. In particular, the central issue is the size of fiscal multipliers when the economy is in recession.The gist of the recent literature on this issue has effectively been to echo earlier Keynesian arguments that government spending is likely to have larger expansionary effects in recessions than in expansions. Intuitively, when the economy has slack, expansionary government spending shocks are less likely to crowd out private consumption or investment. To the extent discretionary fiscal policy is heavily used in recessions to stimulate aggregate demand, the key empirical question is how the effects of fiscal shocks vary over the business cycle. The answer to this question is not only interesting to policymakers in designing stabilization strategies but it can also help the economics profession to reconcile conflicting predictions about the effects of fiscal shocks across different types of macroeconomic models.