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...