This introductory chapter identifies who controls immigration law and how. Many recent immigration controversies in the United States have been cast as power struggles between the political branches. In nearly every clash, the President’s detractors invoke the same supposed constitutional wisdom: that control over immigration belongs in the hands of Congress. When President Donald Trump, just days after his inauguration, signed an Executive Order banning immigration from six predominantly Muslim countries, critics immediately attacked the action as tyrannical, discriminatory, and a violation of the immigration statutes enacted by Congress. Similarly, when President Barack Obama announced that he would shield young unauthorized immigrants from deportation, the backlash was swift. Commentators, members of Congress, and even his own immigration enforcement agents pilloried him for usurping the constitutional authority of Congress. The modern-day battles of the Trump and Obama eras reflect a deep historical truth about the structure of immigration policymaking: that the President stands at its center. This book tells the story of how the President became the immigration policymaker-in-chief. Today, the President’s extraordinary power over immigration policy grows out of his utterly ordinary constitutional duty to enforce the law. The logic and tools of enforcement therefore define contemporary immigration policy, empowering the President and executive officials to shape the meaning of immigration law.