A core challenge for foreign relations law – as with all public law – is how to constrain decision-makers and yet enable them to be effective. In the United States, the original solution was an innovative distribution of powers between Congress and the President. Over time, however, this distribution of powers has changed with respect to foreign affairs. The core story, often told, is of the rise of presidential power to undertake actions without congressional consent. As the President’s power has grown in scope, however, it has also become subject to more procedural requirements. Transparency and regularity are now expected and often mandated with respect to uses of executive power related to foreign affairs. These requirements do not stem from the U.S. Constitution but rather from a web of diffuse sources, including U.S. congressional law, U.S. executive branch practice, and various aspects of international law. This new set of constraints serves to some degree as a substitute for the original constitutional constraints. This book chapter describes these twin developments in relation to presidential foreign affairs power – the erosion of scope-based checks and the rise of process-based checks. It argues that international law has played a role in both developments. In closing, it considers the extent to which the changes described here may have relevance for the practice of other nations.