Once limited to the computing world – specifically, to data centres - energy efficiency has become an issue in telecommunication networks in the recent years, mainly owing to two reasons: the increased attention to environmental aspects, and the growing operational expenditure (OPEX) of Telcos and Internet Service Providers (ISPs). Great attention is currently dedicated to energy-efficient “green” networking in both wired and wireless scenarios. A number of techniques have been identified and are continuously being consolidated and improved to reduce the energy consumption in networking equipment, while maintaining at the same time a desired level of Quality of Service (QoS) – and, ultimately, of Quality of Experience (QoE) in the users’ perception – in dependence of the actual carried traffic load. The range of possible actions spans from the design aspects (re-engineering of both silicon and network architecture to reduce complexity) to the on-line operation at small and large time scales (dynamic adaptation of devices’ energy consumption-performance trade-off and “smart sleeping” techniques). While Green Networking is pacing to become a mature bulk of technologies, many aspects still need to be investigated to harmonize its presence in the complex scenario of the Future Internet.