Unexplored areas in EU integration literature regarding regional representational channels give impetus to the research. Novelty lies in its concentration on reciprocal regional governmental and EU relations to test the hypothesis that Regions with Legislative Competencies (RLCs) interact in EU policy and decision-making to influence it through both formal and informal means. The work focuses on RLCs because they are able to harness a large degree of power and influence vis-à-vis the nation state, which increases their visibility in EU policy. The Basque Country is the case study due to its high degree of autonomy and resources that maximises its ability to influence EU policy. The period of analysis starts from the Maastricht Treaty negotiations (1991-1992) and concludes with the Basque parliamentary elections (2009). Existing literature explores the regional tier emphasising its involvement in EU decision and policymaking, primarily concerning structural and cohesion funds. Other scholarly contributions identify central governments as the regulators of substate regional participation at EU-level policymaking. The thesis argues that multi-level governance (MLG) offers new insights into examining EU integration and the phenomenon of regional ‘circumvention’ of central governments through informal representational avenues, including interregional organisations and diverse lobbying efforts. A MLG approach is useful because it considers varying levels of influence and jurisdiction of substate, state, and supranational entities within the EU. The thesis argues that the effective representational capability of RLCs depends on formal and informal institutional contacts, and it explores the most effective forms of representation for Euskadi and the policy areas where its activism has an impact. Fieldwork consists primarily of informal semi-structured interviews with EU representatives, the central state, Basque institutions, and interregional organisations. They are essential concerns for the methodology because they form a range of informal and formal representation channels, which have not been thoroughly surveyed in the literature.