If the federal government has constitutional power to address a social ill, and hence has power under the Supremacy Clause to preempt state, local, and common law regimes, is there a principled rationale for distinguishing federal standard setting that sets a federal floor or ceiling? At first blush, the two appear to be mere flip sides of the same federal power, only distinguished by their different regulatory preferences for a world of minimized risk (with floors) or higher levels of risk (with ceilings). This Article argues, however, that these two central regulatory choices are fundamentally different. Floors embrace additional and more stringent state and common law action, while ceilings actually are better labeled a unitary federal due to how they preclude any more lax or more stringent action as well as the different actors, incentives, and modalities of information elicitation and proof of common law settings. Advocates of less hindered markets respond that this is precisely the idea-regulatory certainty is enhanced with ceiling preemption, allowing producers of goods to plan with confident knowledge of the regulatory terrain, unbuffeted by an array of uncoordinated actors. This debate was, until recently, largely hypothetical. Recently, however, in settings as diverse as product approvals, to of risks posed by chemical plants, to possible legislation regarding greenhouse gases contributing to climate change, legislators and regulators have suggested or asserted such a broad, preemptive impact. The federal action, whatever it is, would be the final regulatory choice. But under what theory of and legislation can one be confident that locating all decisionmaking power in one institution at one time will lead to appropriate standard setting? In fact, advocates of risk regulation, experimentalist regulation scholars, and, at the other end of spectrum, skeptics about the likelihood of public-regarding regulation, all call for attention to pervasive risks of regulatory failure. Agency and legislative inertia, information uncertainties and asymmetries, outdated information and actions, regulatory capture, and a host of other common regulatory risks create a substantial chance of poor or outdated regulatory choice. Considering these pervasive risks of regulatory failure, the principled distinctions between floor and ceiling preemption become apparent. Vesting all decisionmaking power in one institution can freeze regulatory developments. Unitary federal choice ceiling preemption is an institutional arrangement that threatens to produce poorly tailored and public choice distortions of the political process, whether it be before the legislature or a federal agency. Floor preemption, in contrast, constitutes a partial displacement of state choice in setting a minimum level of protection, but leaves room for other actors and additional regulatory action. Floors anticipate and benefit from the institutional diversity they permit. This Article closes by showing how the institutional diversity engendered by retaining multiple layers of law and regulatory actors creates conditions conducive to reassessment and adjustment of often rigid or outdated regulation.