Paper presented at the Sustainable Places conference, Savona, Italy, 2015 The reduction of carbon emissions in cities is a systemic problem, which involves multiple scales and domains and the collaboration of experts from various fields. The “smart city” approach can contribute to improving the energy efficiency of urban areas provided that there is reliable data –from the different domains concerned with carbon emission reduction– to assess their energy performance and to make decisions to improve it. In the SEMANCO project, we applied Semantic Web technologies to solve the interoperability among data, systems, tools, and users in cases dealing with carbon emission reduction in urban areas. In the OPTIMUS project, the tools and methods developed in SEMANCO are being further enhanced and applied to the development of a decision support system (DSS) to help local administrations tooptimize the energy use of public buildings.