One of the most fiercely debated questions about EU regulatory governance is the respective role played by functional and political factors in regulatory integration. This article contributes to this debate by focussing on the functional factor. Based on a refined conceptualisation of functional stakes, it finds that they vary across sectors, evolve over time, and that these variations are reflected in the degree of regulatory integration observed. When member states perceive regulatory integration as a solution to one of their most pressing problems of the moment, they value – and sometimes even actively push for – the delegation of regulatory powers to the EU. This argument is subject to a credibility probe based on two within-sector analyses of temporal patterns of regulatory integration in energy and telecommunications. The empirical analysis lends support to the conditioning role of the functional factor in the design of EU regulatory governance.