This paper describes various wireless power transmission (WPT) technologies including inductive coupling, radio waves, and resonance coupling. Theoretically, these three WPT technologies are similar, in that all of them depend on Maxwell's equations. However, there are pros and cons for each application of the WPT. Therefore, a suitable WPT technology must be selected for each application. Herein, the theory, technologies, and applications of WPT are discussed. WPT is a useful and convenient technology that can be employed to charge the batteries in mobile phones, notebook PCs, electric vehicles, light-emitting diodes, integrated circuits, and other equipment without the need for a wire connection. For systems that use very low power, the battery can be removed altogether, and the systems can be run on energy harvested from ambient radio frequency and microwave radiation. Therefore, the number of batteries can be reduced when wireless power is available from various locations, and because batteries can be charged wirelessly, concerns about the shortage of batteries can also be reduced. WPT via microwaves, for example, can be applied in the future to stable and CO2-free space-based solar power satellites. Overall, WPT will support both future energy production and the environment.