The European Central Bank (ECB) has been conferred exclusive competence in banking supervision and enjoys significant powers in leading banking supervision within the Single Supervisory Mechanism (SSM) in the euro area. Such leeway for action is essential for efficient decision-making and steering ‘ongoing’ supervision; while the national competent authorities act in a decentralised implementation framework. Yet the SSM as a system is not fully integrated – institutionally, administratively, in its still evolving governance – and therefore cannot be considered single despite its name. The SSM operates nonetheless within shared and interlocked legal orders, exhibiting some features of cooperative federalism but remaining subject to evolving centripetal and centrifugal forces. An integrated system for banking supervision has an institutional and substantive dimension. I examine the SSM supervisory architecture, institutional organisation, and its governance with a legal and contextual approach. The position of the ECB is outstanding in leading direct banking supervision and in its oversight over the system, including an expanding normative power. Nonetheless, the preparation, implementation and execution of supervision may be exercised in different instances, including through joint structures. In a context of an unharmonized regulatory framework and banking markets fragmented along national lines, how can banking supervision be achieved efficiently in the Banking Union? I define the concept of efficiency as including qualitative and adequacy aspects. Qualitatively, banking supervision should be consistent, uniform, and harmonised in the system. Adequate supervision should attain the SSM objectives through the use of (limited) resources, proportionally. I conclude that applying proportionality and sincere cooperation, general principles of law, and a governing principle of consistency should sustain the integrity of the system. The SSM must keep and develop cooperative and incentivising mechanisms to pursue its objectives – banks’ safety and soundness, the stability of the financial system – in the interest of the Union as whole.