Future wireless networks aim to provide high-energy efficiency, high spectral efficiency, low cost, and support a massive number of connected devices in prospect the evolution of Internet of Things (IoT) and smart cities. This calls for an immediate need to inspect sustainability in future wireless networks, and how can these networks be energy-efficient and environmentally friendly. The goal of this article is to investigate the role of future communication systems in building sustainable and energy-efficient smart cities. Ano verview of 5G / 6G networks, their KPIs, and use cases will be presented. This is followed by discussing the use cases served by these networks, green 5G / 6G technologies, and sustainability indicators. We will discuss the recent research studies on green technologies such as energy harvesting and their sustainability measures across the economic, environmental, and social dimensions. Finally, we will identify potential solutions and research directions to optimize sustainability in future wireless communications. 2022 IEEE. M. Shehab and T. Khattab are with the Department of Electrical Engineering, Qatar University, Doha, Qatar. Murat Kucukvar is with the Department of Industrial and Systems Engineering, Qatar University, Doha, Qatar. D. Trinchero is with Dipartimento di Elettronica, Politecnico di Torino, Torino, Italy. This research work was made possible by grant number AICC03-0530-200033 from Qatar National Research Fund, QNRF (a member of Qatar Foundation, QF). The statements made herein are the sole responsibility of the authors. Scopus 2-s2.0-85136131919