We use a dynamic model of the …rm to ascertain both the value and the determinants of the tax bene…ts of debt. A standard parameterization suggests that the value of the interest tax shields represents around 3% of …rm value, and it varies considerably across industries. In addition, our results show that this component of the stock price behaves countercyclically over the business cycle. Finally, besides the interest rate on debt and the corporate income tax rate, we …nd that the curvature of the production function is one of the most important determinants of the tax advantage of debt.JEL classi…cation: G31, G32