The development of wireless communication networks worldwide has triggered a massive growth in the number of wireless communication devices and sensors for applications such as logistics and transportation, environmental monitoring, energy management, and safety management, etc. It is expected that in the era of the Internet of Things (IoT) in 2020, there will be 50 billion wireless communication devices connected together worldwide with a connection density of 1 million devices per km2. However, battery-powered wireless communication devices have limited energy storage capacity and their frequent replacement can be costly, cumbersome, or even impossible, which creates a serious performance bottleneck in realizing reliable and ubiquitous wireless communication networks. Wireless power transfer (WPT), offers a viable solution to facilitate efficient and sustainable communication networks to serve energy-limited communication devices. In this book, the authors discuss the circuit design, resource allocation algorithm design, and protocol design to tackle the above challenges by both theoretical and practical approaches which will bridge the gap between theory and practice, with long-term contributions to the evolution of future sustainable WIPT networks.