Surface flashover of insulators at dc voltage is one of the key issues limiting the stable operation of electric power equipment and electronic components both in pressurized gas and vacuum. In this article, the history and key issues of the surface flashover of insulators at dc voltage in compressed gas and vacuum conditions are reviewed, based on which the influencing factors of the surface flashover at dc voltage in compressed gas and vacuum conditions are discussed. The theoretical models of dc surface flashover are summarized. Finally, the related problems of surface flashover at dc voltage are concluded, and future work is prospected.