This document is the first whitepaper of the 6G Smart Networks and Services Industry Association (6G-IA) dedicated to the technological questions of sustainability of future mobile telecommunications systems (6G). Produced as a collaborative effort within the 6G-IA Vision and Societal Challenges Working Group (VSC WG), it results from both internal and collaborative research activities of the involved partner organizations, most notably within the research projects of the Smart Network and Services Joint Undertaking (SNS JU).This whitepaper identifies main challenges in the area of operational Sustainability of 6G by contrasting the consensual 6G vision of the European Industry and the expected evolution of services and the mobile ecosystem with the lessons learnt from 5G, in the sense of the main energy consumers and the reasons for the latter. Trying to address these challenges, it identifies several candidate enabling technologies and more general approaches for energy consumption and carbon dioxide emission reduction and possible current research and standardization gaps, to be considered in the future work on the way towards more sustainable 6G.