There is widespread confusion both in policy circles and in the academic literature about how to measure the progressivity of a tax change. The confusion is particularly vexing because policymakers and analysts often rely on progressivity as a guidepost in constructing and analyzing policy, but do little to justify the particular progressivity measures that they employ. Progressivity measures—which can differ considerably from one another—tend to be picked haphazardly or chosen based on arguments that have rhetorical flair but lack normative substance. Thus, policy is being constructed and evaluated based on distributional measures that may not be meaningful and, in fact, may be misleading. This Note proposes a framework for analyzing measures of progressivity. In particular, if the measures are to gauge accurately changes in tax fairness, progressivity measures must be rooted in whatever theory of distributive justice motivates our concern for distribution. This Note applies this approach and draws connections between particular measures of progressivity and individual theories of distributive justice.