This Recommendation: Describes the necessity for energy saving; Reviews potential technologies; Identifies multiple viewpoints to be considered; Identifies major functions and their cyclic interactions; Analyses possible impacts by introducing the technologies, and Itemizes high-level requirements to achieve energy saving of Future Networks (FNs). The framework and ideas described in this Recommendation may also be applicable and useful for networks which are not categorised as Future Networks, although the description is produced through full recognition of the objectives and design goals of Future Networks [ITU-T Y.3001].