This paper develops a new-Keynesian model with nominal depreciation allowances to consider the effects of temporary tax-based investment incentives on capital spending and real activity. In particular, we investigate the effects of a temporary expensing allowance on investment in partial and general equilibrium and challenge the conventional view, advanced by Auerbach and Summers (1979) and Judd (1985), that partial-equilibrium analyses overstate the calculated impact of such policies. We also explore two additional questions. First, we investigate a claim noted by Auerbach and Summers and analyzed by Christiano (1984) that such incentives can be destabilizing. Second, we consider the relative impact of two types of tax-based investment incentives: a temporary partial-expensing allowance and a temporary reduction in capital taxes.