This report describes the process for the definition of EU energy transition pathways developed in REEEM and introduces the structure, main assumptions and preliminary impact indicators of the first pathways defined. It addresses different groups of readers, specifically 1) the general audience, since it presents in a transparent fashion the methodological and modelling framework developed in REEEM, 2) the European Commission, through its explicit link with the Energy Union and the SET-Plan, 3) energy modellers and modelling projects, in that it delivers a scientifically sound and repeatable process to define pathways for a multi-model and multi-sectoral analysis, 4) stakeholders, since it pinpoints the steps in the REEEM activities where interaction with them is essential and explains how it occurs, 5) the REEEM partners, in its guiding the modelling teams towards the creation of an integrated modelling framework and the continuation of the quantitative analyses. The methodology and this related report are a joint effort of all the partners involved in REEEM Work Package 1 (Transformation strategies and pathways).