Wireless power transfer has recently become a hot research topic due to its potential wide applications in different areas such as implanted medical devices and personal electric devices.This paper presents a review of the progress in the area.Firstly,the backgrounds and basic theories of general wireless power transfer technologies are introduced.Then one of the technologies,the magnetically-coupled-resonant system,is presented and focused on.Properties of the magnetically-coupled-resonant system are described,including over-,critical-and under-couplings.Finally,challenges in developing and applying the magnetically-coupled resonant wireless power transfer systems are summarized.