The development of power systems and the move to smart grid have increased the need for new technologies. In this regard, solid-state transformers have been proposed as a suitable alternative to conventional transformers. Solidstate transformers are among the equipment based on power electronic converters that in addition to better performance than conventional transformers provide a variety of other services. In this article, the concept and types of solid-state transformer topologies and configurations and their applications, especially in smart grid, are investigated. Studies show that the various characteristics of solid-state transformers have led to much consideration as potential transformers in smart grid applications, the integration of distributed generation sources, modern traction systems, and so on.