This book highlights the interaction between science and politics and between research in economics and European Union policy-making. It focuses on the use of Quantitative tools, Top-down and Bottom-up models in up-stream European decision-making process through five EU policy case studies: energy taxation, climate change, energy efficiency, renewable energy, and internalisation of external costs. The author reveals how the European Commission grounds part of its legitimacy on the «objectivity of the figures» and on its «technical charisma». Faced by strong stakeholders, an elected European Parliament and a Council representing the national interests, the Commission defends the credibility of its policy initiatives (e.g. energy and environment targets, new market mechanisms) by scientific reports. Through an in-depth analysis of the preparatory legislation (Commission White Papers, Communications, Directive Proposals, Staff Working Documents, Impact Assessments), the author explains how the European Commission justifies economically and supports politically its initiatives. This book offers a new way of understanding the EU decision-making process with special reference to the energy and environment fields.