This Article proposes a framework for analyzing how private-sector legal interpretations influence public policy. Political scientists and legal scholars use the terms “bureaucratic drift” and “legislative drift” to describe how administrative agencies and future legislative coalitions affect public policy enacted by Congress. This Article identifies a third category of policy drift: “planning drift.” Planning drift describes deviations from an enacting legislature’s policy preferences that result from private experts’ interpretations of existing law. After Congress enacts a statute, the first people to interpret and apply the new legislation generally are not regulators or judges, but instead are private experts, such as lawyers, acting in the service of their clients. Although these experts’ interpretations do not have legal authority in a formal sense, this Article elaborates mechanisms through which these interpretations shape the course of public policy. Specifically, these interpretations give private experts a first-mover advantage in the interpretation of new legislation and affect the substance of subsequent legislative and bureaucratic interventions. Where private experts’ legal interpretations distort legislative policy preferences, Congress may have an incentive to limit planning drift. From Congress’s perspective, however, planning drift is not always undesirable. As this Article argues, Congress may tolerate or even welcome planning drift, depending on how it interacts with other types of policy drift and congressional politics and purpose. This Article concludes by outlining strategies that Congress could use to constrain planning drift where it is not desired. In an appendix, this Article illustrates planning drift using a detailed historical case study of the rules restricting “trafficking” in corporate tax losses.