"""Efficiency First"" (E1st) is a fundamental principle applied to policymaking, planning and investment in the energy sector, which is gaining visibility in European energy and climate policy. ENEFIRST will help making the E1st principle more concrete and operational, better understand its relevance for energy demand and supply and its broader impacts across sectors and markets, focussing on the buildings sector. Its main objectives are to define the principle of E1st in practical terms, assess how it has been applied internationally and how it applies to the EU context, to assess the value of applying E1st across different policy areas for buildings' end-use energy efficiency and to quantify the impacts of increased building energy efficiency for the future energy system in the EU, and to identify key policy areas for the application of E1st and develop policy proposals for its implementation in the EU Buildings Sector. The methodology of ENEFIRST has been designed in light of the novelty of the approach informed by a deep understanding of the current energy policy landscape and the existing practices of energy efficiency policy making and is based in three pillars a) identification of the most relevant policy areas where the E1st principle can be applied to achieve the highest impact in terms of energy system benefits, through screening 5 areas of potential application, modelling three scenarios and five modelling case studies and applying the E1st in three regions in the EU, b) application of the E1st in policy instruments, through assessing the applicability and transferability of international E1st approaches and quantifying the impacts of E1st (through modelling and multi-criteria analysis), and c) application of E1st through the design of policy instruments and the analysis of their application in country case studies. All the foreseen tasks will be carried out with the validation of experts' groups that formulate the ENEFIRST Stakeholder Community."