Collegiality is a core legal principle of the European Commission's internal decision-making, acting as a safeguard to the Commission's supranational character and ensuring the Commission's independence from EU Member States. Despite collegiality's central role within the Commission, its legal and political implications have remained critically underexamined. Collegiality in the European Commission sheds light on this crucial aspect of the Commission's work for the first time. In this novel study on collegiality, Maria Patrin proposes an innovative framework for assessing the Commission's institutional role and power. The book's first part legally examines collegiality, retracing collegial procedures and actors in different layers of decision-making — from the Commission's services to the College of Commissioners. The second part of the book explores the implementation of collegiality through illustrative case studies, focusing on various Commission functions including legislative initiative, infringement proceedings, and economic governance. Partin's empirical analysis unveils a disconnect between the legal notion of collegiality and its concrete application in institutional practices. These variations raise normative questions on how to ensure the unity of the Commission as a collegial body despite the diversification of decision-making functions. They also invite a re-examination of the Commission's multifaceted role in the current EU institutional, legal, and political setting. Adopting an interdisciplinary approach that delves into both the legal substance and the political-institutional practice of collegiality, this book offers a unique, behind-the-scenes insight into the Commission's decision-making processes, furthering our understanding of the EU's institutional system.