This chapter explores three important themes in constitutional immigration law. First, as creators and executors of U.S. immigration policy, Congress and the President have virtually limitless power over designating who may enter the country, under what terms, and when they must leave. Second, this plenary power over immigration law was created by a complicit U.S. Supreme Court and has never been constitutionally repudiated. The plenary power doctrine thus enjoyed the Court’s imprimatur, guaranteeing considerable political branch latitude even if laws resulted in systemic exclusion based on invidious criteria like race or national origin. Third, notwithstanding this historical deference to the political branches, the Court has provided an occasional yet essential check on legislative and executive overreaching.