This article presents a new measure of the efficiency consequences of tax policies and explains how this new measure can shed light on a wide range of tax law debates. We build upon the “elasticity of taxable income” approach pioneered by public finance scholars over the last quarter century and extend that approach to address complex tax systems with multiple rates, multiple bases, and administrative and compliance costs. The resulting measure—the behavioral elasticity of taxable revenue, or BETR—captures the change in real resources available to society caused by any marginal change in tax rates, the tax base, or tax enforcement. We argue that the BETR can serve as a guide to a broad array of tax policy puzzles, and we illustrate the BETR’s utility by applying it to questions such as the proper treatment of mixed personal/business expenses and the optimal mix of audits, reporting requirements, and penalties. We also consider the relationship between the BETR and the distributional aims of tax law. While the BETR is a measure of efficiency and not distribution, the BETR can aid policymakers in deciding both how much to redistribute and how to accomplish distributional objectives most efficiently. We end with reflections on the implications of the BETR for the design of non-tax legal rules.