This book explores discretionary decision-making in migration systems in Europe. It investigates how European law influences the existence, exercise and effects of discretion. Firstly, it argues that national lawmakers often regard discretionary decision-making as the most viable option to accommodate standards of European law in their national migration systems. Secondly, the book shows how the exercise of this power has changed from an area of free choice to an instance of legally instructed decision-making. 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.