Book excerpt: This book covers the theory, modeling, and implementation of different RF energy harvesting systems. RF energy harvesting is the best choice among the existing renewable energy sources, in terms of availability, cost, size, and integration with other systems. The device used for harvesting RF energy is called rectenna. A rectenna can work at the microwave, millimeter-wave, and terahertz waves. It also has the capability to operate at optical frequencies to be used for 6G and beyond communication systems. This book covers all aspects of wireless power transfer (WPT)/wireless energy harvesting (WEH), basics, theoretical concepts, and advanced developments occurring in the field of energy harvesting. It also covers the design theory for different types of antenna, rectifier, and impedance matching circuits used in RF energy harvesting systems. Different future and present applications, such as charging of vehicles, smart medical health care, self-driven e-vehicles, self-sustainable home automation system, and wireless drones, have also been discussed in detail.