This chapter describes the most significant institutional and policy changes that the banking union has introduced and demonstrates why this represents a rupture with the past. It analyzes the impact of banking union and considers the wider ramification for the European Union's (EU's) single market. The chapter discusses the major claims of political science literature for explaining the emergence of the banking union on the European agenda and the actual policy outcome. The 2008 banking crisis led in 2010 to the creation of a European Banking Authority as part of a policy shift to adopt a single rulebook and enforce European standards more vigorously. For the development of the EU itself the German argument that Treaty change is needed for making the banking union more solid merits serious consideration. The chapter concludes by outlining questions for further research and policy developments, and argues that Treaty changes are necessary to build the banking union on a firmer political and legal foundation.