This thesis explains the adoption of the Climate & Energy Package (2008), the Third Package (2009) and the Security of Gas Supply Regulation (2010). These three politically highly sensitive pieces of legislation are at the heart of the EU’s first-generation climate and energy policy which was established between 2007 and 2010. The climate & energy legislative files were adopted as an early agreement (EA), meaning via an informal compromise between the EU’s co-legislators (i.e. Council and Parliament) and after one rather than three possible readings. The adoption of these files under co-decision rules in such a short timeframe was remarkable as the bargaining process between the co-legislators was marked by collective action problems, disagreement over which set of rules to apply and had to be carried out under urgent circumstances caused by (inter)national events and/or regimes. This thesis applies a ‘thick’, rational choice institutionalism bargaining model in explaining the speed of the legislative decision-making process on the three pieces of legislation. Next to interests, this model incorporates ‘ideas’ meaning principles and norms found in EU primary and secondary legislation. In legislative decision-making, the EU puts a premium on the ability to provide convincing ideas or ‘focal points’ around which actors’ interests can converge and which promote common action. The cumulative message of the research conducted on the three files is that ideas served as instrumental tools in sustaining cooperation and cementing an acceptable compromise solution among a set of players with divergent interests.