This thesis has examined Britain’s continuous support for the EU’s enlargement process in the period from 1975 to 2014 utilising a Liberal Intergovernmentalist (LI)perspective. The research has mainly confirmed the usefulness of LI as a theory, especially its conception of national preference formation and its two-level depiction of the EU decision-making process. However, the findings below also highlight some challenges for LI. Enlargement has continually proved to be a complex issue, which significantly constrains the ability of governments (including successive British governments) to make decisions about it using a rational, cost-benefit analysis. LI gets around this problem by arguing that in such cases of complexity, national policy-makers may fall back on ideological or geopolitical preferences and arguments for enlargement, and the evidence from the case studies below, confirm this point. But this ‘multi-causal’ approach also appears to both undermine LI’s parsimony as a theory, and to raise questions whether it is, in fact, capable of anything more than a ‘thick description’.