It is widely accepted that the correct discount rate for the tax shield depends on whether the value of the debt is a fixed amount or is a proportion of the value of the firm. 1 It is simply assumed that the appropriate discount rate for the tax shield is the cost of debt because the amount of debt at the beginning of the period is known. In this pedagogical note, using a simple two period numerical example, I will demonstrate that the correct discount rate for the tax shield does not depend on whether the value of the debt is a fixed amount or is a proportion of the value of the firm. 2 It is always correct to use r, the return on unlevered equity, to discount the tax shield.