This chapter provides an analysis of the role of the European Commission and the member states in governing differentiation in European integration - both historically and vis-à-vis the ‘Brexit process’. It ultimately asks how the balance of powers of the Commission and the member states in this regard could evolve in the future. Whereas the administrative capacity of the Commission’s structure and administration are crucial for the implementation of differentiated integration, it is only the EU’s member states’ governments which can provide sufficient impetus - if not legitimacy - for triggering differentiation and thus shaping the future design of the EU. This chapter explores how the institutional dynamics in processes of differentiation have developed over time, assesses the prospective roles that the Commission and the member states could assume in triggering further differentiation in a ‘post-Brexit’ Union, and eventually proposes that coordination and cooperation between the Commission and governments of member states are likely to increase in the future.