Concentrating solar power (CSP) is a mainstream of solar energy utilization, and thermionic emission is a potential way to convert concentrated solar radiation into power with a theoretical efficiency of 50-70%, surpassing both Shockley-Queisser limit and photo-thermal limit. This literature attempts to provide a comprehensive understanding of and an insight into solar thermionic energy conversion. The fundamentals of electron emission from electrodes and electron transport in vacuum gap are presented, as well as the state of the art of solar thermionic energy conversion technologies, including heat-induced thermionics and photon-enhanced thermionics. The former is driven by thermal energy, whereas the latter takes advantage of both quantum photon energy and thermal energy. Burgeoning research indicates that photon-enhanced thermionic conversion is a promising technology for concentrating solar power due to the high efficiency and simple operating mode. Now, it is important to develop novel materials and coating technologies to facilitate electron emission and reduce space charge effect in interelectrode vacuum. Structural design of thermionic converters and top bottom configuration of solar-electricity systems are suggested for practical applications.