This report aims to synthesise the work carried out in REEEM, to extract the main messages from the project activities and to highlight the main impacts. It reports on the main objective of the project, which is to provide a comprehensive understanding of the system-wide implications of energy strategies in support of transitions to a competitive low-carbon EU energy society. This report constitutes an updated version of the midterm report D1.2a – First Integrated Impact Report. It reflects methodological discussions extensively taken under D1.1 - Report on pathway definition, and it summarises the findings of the studies presented in all other project deliverables (Case study reports, Focus reports and sectoral Policy Briefs). Section 1 sets the background: it focuses on the EU’s strategic priorities for the energy system, identifies the challenges related to such priorities and summarises the contributions of key EU-funded actions to informing such challenges. Such funded actions represent past or present efforts which the project partners have collaborated with. The aim of the REEEM Consortium in this respect has been to leverage on and complement the findings of such actions. Section 2 describes the design of the REEEM pathways: a set of scenarios specifically co-designed with stakeholders, to explore the above challenges and provide insights complementing those of other research. Section 3 describes the key messages obtained through the integrated modelling framework, providing policy relevant insights into the challenges and opportunities of decarbonisation across economic sectors, wider society and the environment. Section 4 synthesises the messages from Section 3 into 7 consistent, integrated messages which summarise the findings of the REEEM project. Those integrated messages are: 1. Impacts of the transition to a low carbon EU energy system are multi-dimensional and spatially varied 2. Broader engagement is imperative for deep decarbonisation 3. The EU low-carbon transition is strongly linked to non-EU drivers 4. There are non-trivial multidimensional path dependencies that cannot be ignored 5. Among the technology trends, energy efficiency and electrification of transportation are consistently confirmed as potential enablers of the decarbonisation 6. Focusing on direct mitigation misses important leakage effects 7. New energy security paradigms Moreover, Section 4 describes the different tools and activities developed under the scope of REEEM, aiming at communicating the findings and promoting engagement. Finally, Section 5 presents how the messages may be relevant for and communicated to various stakeholders of the transition, while suggestions on how to build on the current research and make further progress are made.