Cities are expected to play a key role in the implementation of Europe 2020 strategy. The city authorities need to lead relevant actions towards energy-efficient neighborhoods. Engaging city authorities in this respect is of paramount importance.Although there are plenty of energy and other related data sets available in city level, their appropriate integration for local authorities' decision support remains a challenge. Indeed, there are no approaches and respective systems using multidisciplinary data sources to analyze and integrate them in a smart way, towards cities' energy optimization.The overall vision of the project "OPTIMising the energy USe in cities with smart decision support systems (OPTIMUS)" is the design, development and implementation of a Decision Support System (DSS) addressed to city authorities, in order to assist them to optimize the energy use in their premises and reduce CO2 emissions.OPTIMUS will provide an integrated ICT platform that will collect and structure open data sets from five domains (namely: weather conditions, social mining, buildings' energy profiles, energy prices, energy production). Moreover, OPTIMUS will use semantic/intelligent technologies so as to combine them and propose energy optimization plans. The resulting web-based DSS will be validated through pilot applications in three different cities: Savona (Italy), Sant Cugat del Vallès (Spain) and Zaanstad (The Netherlands).OPTIMUS will have, by design, the necessary degree of generalization so as to be easily adapted to cities with different characteristics. This is related to another goal of the project, i.e. to produce an advanced and intelligent turn-key solution addressed to any city that has as purpose to implement and monitor sustainable energy action plans.