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This paper reviews the energy efficiency of optical-packet-switching (OPS) systems in comparison with electronic packet switching and hybrid packet switching in the context of future networks. The paper will first discuss the energy efficiency metrics that should include considerations for life-cycle analysis, applications, and network-wide goodput. The state-of-the-art electronic packet switching router is currently energy-limited in scalability as it is difficult to implement a router with more than 1 MW power consumption. The OPS router that imitates electronic router's store-and-forward schemes is expected to suffer poor energy efficiency due to the complexity in the high-speed control plane necessary to control many optical buffer stages. The hybrid optical router achieves easier buffer management but its energy efficiency is still limited by the store-and-forward approach. The OPS router based on all-optical contention resolution without relying on store-and-forward method can keep the control plane very simple and achieve very high energy efficiency. Network-wide performance and energy efficiency in the context of generalized multiprotocol label switching (GMPLS)- and multiprotocol label switching (MPLS)-based networking are also discussed.