Europe can become the leader in CO2 emission reduction by applying innovative solutions to its built cultural heritage. According to the European Recovery Plan one of the actions that needs to be taken to tackle the current crisis, is investing in energy efficiency. Historic urban buildings consume 4% of all energy and are responsible for 3% of CO2 emissions. Therefore, improving energy efficiency in historic buildings and historic districts is essential.Nevertheless, most of the current developments in energy efficiency address new construction without dealing with the unique problems of historic structures. A number of technologies and products have been developed, however many of the solutions are not acceptable for historic structures due to the necessity of preserving integrity and authenticity.Therefore, the main goal of EFFESUS is to develop and demonstrate, through case studies a methodology for assessing and selecting energy efficiency interventions, based on existing and new technologies that are compatible with heritage values. A Decision Support System will be a primary deliverable. The environment in historic buildings and urban districts is controlled differently from modern cities and accordingly the project will also develop a multi-scale data model for the management of energy. In addition, new non-invasive, reversible yet cost-effective technologies for significantly improving thermal properties will also be developed. Finally, existing regulations and building policies may not fit cultural heritage specificities so the EFFESUS project will also address these non-technical barriers.These outcomes will be achieved through 10 work packages, performed by an interdisciplinary consortium of 23 partners from 13 countries. Due to the attractiveness of this niche market, 36 % of the project budget is allocated to SME’s, which will work together with large companies, research institutions and end users throughout the duration of the project.