Smart city applications require pervasive and large-scale infrastructures, which include heterogeneous IoT devices and distributed information systems, thus posing interoperability and cost challenges. Interoperable solutions, exploiting fog/edge/cloud computing resources, are fundamental for fair competition, especially in public procurements, while costs savings are necessary to speed up the smart city innovation pace, by enabling more stakeholders to easily enter the market, especially SMEs. The Fed4IoT project faces the interoperability issue, focusing on large scale environments and addressing the problem at different and synergic levels: device, platform and information. The goal of the project is “Federating IoT and Cloud Infrastructures to Provide Scalable and Interoperable Smart Cities Applications by introducing novel IoT virtualization technologies” and will be pursued through the following steps: 1) select/integrate/improve existing IoT and cloud platforms, including oneM2M, FIWARE and 5G ETSI MEC, so as to establish a reference interoperability solution; 2) use such reference solution to build up a pool of federated IoT and fog/edge/cloud resources; 3) design novel device-level IoT virtualization technologies to create "IoT slices" formed by virtual IoT devices and computing resources, exploiting the federated resource pool; 4) support orchestration and programmability for optimal IoT virtual function deployment and Big Data processing; 5) integrate information coming from different IoT domains and other city sources; 6) integrate the system components. The project solutions will be technically validated by implementing four specific smart city applications, based on a federated EU/JP platform, deployed in real life systems in two EU and two JP cities. The Fed4IoT consortium will also actively support standardization activities (ETSI, oneM2M, ITU, ISO, etc.) and EU/JP initiatives (e.g., AIOTI and ITAC), where consortium members are already involved.