This book explores discretionary decision-making within different national legal systems, relating to European migration law.Firstly, it argues that national lawmakers often view discretion as a necessary tool to accommodate the influences of European law in their national migration systems. Secondly, the book shows how the exercise of this power has changed. And ultimately, it illustrates how both the proliferation and transformation of discretionary decision-making have improved the legal position of migrants in the European Union, providing a compelling interpretation of a key field of EU law.