This thesis asks how the framing of policy issues in EU legislative politics influences the way issues are processed, how it affects which interests play a role during policy drafting and deliberation, and what type of political conflicts and coalitions emerge as a result. Focusing in particular on the European Commission’s role in EU policy-making, this thesis goes on to investigate how actors in EU politics define and redefine the issues at stake according to their shifting policy agendas and in doing so attempt to shore up support and marginalise political opposition. Drawing on the empirical investigation of two decades of EU biotechnology policy-making, the thesis finds that the framing of policy issues systematically affects how the complex and fragmented EU political decision-making process involves or excludes different sets of actors and interests from the diverse political constituencies of the Union. It argues that the Commission’s role in structuring the EU policy space can at times be substantial. Yet the longitudinal perspective adopted in this study also reveals how the structuring and restructuring of the biotechnology policy space led to the increasing politicisation of the EU decision-making process. Eventually, the empirical investigation concludes, the Commission was unable to control the political dynamics set off by the reframing of the policy choices, and the resulting revision of the EU biotechnology policy framework ran counter to the Commission’s original policy objectives. This study thus provides fresh insights into the dynamics of policy-level politicisation and its effects on political conflict and competition in the EU. The framing perspective allows students of EU politics to trace how political agents and institutions interact to shape and at times exploit the complexities of EU policy-making in pursuit of their often conflicting agendas. Finally, the findings suggest that the key to conceptualising the scope of Commission agency in terms of systematic policy dynamics lies in exploring the interlocking effects of policy framing and EU politicisation in the political construction of interests at the supranational level.