This contribution provides an overview of IoT embedded in the cities, its contributions and the challenges, from the environmental perspective. Energy, which is the essential lifeline of smart cities, can be a crucial sector benefiting from IoT implementation. However, there is still a research gap on the offset/saving versus the footprint of the IT sector. The overall reduction of energy consumptions through the integration of IoT is yet to be verified. The end of life management remains an issue. Continuous research and development, as well as assessment, could bring the sustainable smart concept into its real-life implementation in the foreseeable future. The presentation stems to provide a platform for the discussion of future research directions and priorities.