The REEEM project brings together 11 European modelling institutions (among which KTH, UCL, DTU, Universität Stuttgart, KIC Inno Energy and others) under the modelling effort of Horizon 2020 LCE21 call. The 42 months long project, which started in February 2016, aims to gain a clear and comprehensive understanding of the system-wide implications of energy strategies in support of transitions to a competitive low-carbon EU society. Comprehensive technology impact assessments will target the full integration from demand to supply and from the individual to the entire system. It will further address trade-offs across society, environment and economy along the whole transition pathway. To achieve these objectives, pathways for the European energy system to 2050 will be assessed through a plethora of models, soft- and hard-linked with each other. OSeMOSYS will be at the core of a set of enabling tools designed to disseminate and actively engage stakeholders, including a Pan-European (EU28 + Norway and Switzerland) Open-source Engagement Model.