The antenna is one of the most critical components in wireless charging systems since the power transfer efficiency of the system is largely dependent on the antenna performance. In this chapter, the modeling and analysis of antennas are introduced with a focus on increasing the power transfer efficiency. The relationship between wireless power transfer efficiency and antenna parameters like antenna geometry, ohmic loss, impedance matching circuits, and distance between transmitting and receiving antennas is studied analytically and numerically by using circuit theory and full-wave electromagnetic analysis, in order to provide some fundamental and theoretical knowledge to develop antennas for highly efficient wireless charging systems. Finally, challenges and recent studies of antennas for wireless power transfer systems, as well as standards on this field, are briefly introduced.