Recent years have witnessed the growth of mass-marketed tax avoidance schemes aimed at the middle (not top) of the income distribution, with significant implications for tax revenue. We examine the consequences, for the structure of income tax, and for tax authority anti-avoidance efforts, of tax avoidance of this type. In a model that allows for both demand- and supply-side considerations, we find that (1) there is an endogenous threshold income below which taxpayers do not avoid, and above which they avoid maximally; (2) the per-dollar price of tax avoidance is decreasing in income under progressive taxation; (3) endogenous adjustments in the price of avoidance make supply less responsive to anti-avoidance activity than thought previously; and (4) that avoidance may drive a non-monotone (Laffer) relationship between tax rates and tax revenue. The findings suggest that new approaches to anti-avoidance, beyond legal enforcement, may be needed.