With the fast growth of heterogeneous low-cost and high-end mobile devices, there is a need for green designs for ubiquitous development of Internet of things (IoT) due to both health and environment concerns. Unlike other energy harvesting techniques, radio frequency (RF) energy harvesting offers controlled and predictable energy replenishment, which can aid meeting the quality of service requirements of machine-to-machine (M2M) communications. This chapter evaluates the major challenges on the feasibility of RF-powered sustainable M2M communications in 5G mobile technologies and state-of-the-art research toward their practical implementation. Strategies for improving the RF energy transfer efficiency to realize the perpetual operation of IoT are also discussed.