This paper provides guidelines for evaluating the appropriateness of 23 different valuation methods for estimating the present value of tax shields. We first show that the value of tax shields is the difference between the present values of two different cash flows with their own risks: the present value of taxes for the unleveraged company and the present value of taxes for the leveraged company. This implies, as a first guideline, that for the particular case of a perpetuity and a world without costs of leverage, the value of tax shields is equal to the tax rate times the value of debt. The value of tax shields can be lower when costs of leverage exist. In that case, we show that, since the existence of leverage costs is independent of taxes, a second guideline for the appropriateness of the valuation method should be that the value of tax shields when there are no taxes is negative. We then look at the case of constant growth and derive similar conclusions.