Recent legal and economic scholarship has recognized that the government can use structural systems as an efficient way to reduce prohibited behavior. The federal tax system employs structural mechanisms, such as withholding taxes, to foster compliance. The use of structural systems to reduce tax evasion need not be limited to tax administration, however. The Article argues that substantive federal income tax law can--and in many contexts does--foster compliance by harnessing the structural incentives of third parties. Although this phenomenon has gone largely unnoticed, third parties are routinely used by the tax system to verify the bona fides of taxpayer claims in diverse contexts involving reimbursed amounts and other receipts. Yet, third parties do not always behave in ways that are helpful for tax enforcement. The Article therefore identifies contexts in which a third party may have an incentive to collude with the taxpayer. The Article argues that these contexts are ones that the government needs to scrutinize closely and, in certain cases, obstruct with legislation. By contrast, the government can afford to free ride on the incentives of a third party in contexts in which the transfer of funds from the third party to the taxpayer is a zero-sum game.