Abstract: Thermoelectric (TE) devices enable the direct conversion between thermal and electrical energy, and have been widely applied across various industries. Theoretically, they exhibit multiple operational modes based on the current direction and heat flow direction at both sides of TE devices, including thermoelectric generator mode, thermoelectric cooler mode, thermoelectric dissipator mode, thermoelectric thermal resistor mode, thermoelectric heater mode, etc. When considering transient applications of TE devices, even more operational modes emerge. Currently, applications of TE devices primarily focus on thermoelectric generator and thermoelectric cooler, which have undergone systematic investigations. However, research on other operational modes remains underdeveloped. This paper reviews the operational characteristics, performance evaluation parameters, and common applications of various operational modes of TE devices from the perspectives of steady state and transient state based on thermal engineering considerations. It also reviews the efficiency models of TE devices and discusses their future development directions.