This Article starts by analyzing the conventional wisdom, crystallized in the Ninth Circuit’s 1983 decision in Gonzales v. City of Peoria, that state and local law enforcement officers do not require express federal authorization to make arrests for criminal violations of federal immigration law. This view, I explain, is based on overreliance on the line between civil and criminal. Even if a state or local arrest for an immigration crime still leaves federal prosecutors with substantial discretion not to bring criminal charges, it is highly likely that the federal government will force arrestees to leave the United States through the civil removal system, where much less discretion has been exercised. In immigration law, the discretion to arrest has been the discretion that matters. As long as this remains true, state and local arrest authority for immigration crimes reflects assumptions that have the potential to supersede much federal control over immigration enforcement. This consequence of state and local arrests assumes great practical importance when the lessons from Gonzales are applied to federal programs—such as § 287(g) agreements and Secure Communities—in which state and local nonimmigration arrests expose noncitizens to federal immigration enforcement. Though federal decisionmakers may exercise greater and more regularized discretion in response to a larger state and local role, such federal discretion will be fundamentally reactive. Any federal policy that allows state and local governments to be gatekeepers—to permit state and local priorities to decide which noncitizens will be exposed to federal immigration enforcement—risks abdication of federal authority over immigration.