Abstract: Thermoelectric devices, which enable direct conversion between thermal and electrical energy, have been widely employed in fields such as waste heat recovery and medical health. Depending on the direction of the electric current and the heat flow at the two sides, these devices can theoretically operate in multiple modes, including thermoelectric generation mode, thermoelectric cooling mode, thermoelectric dissipation mode, thermoelectric thermal resistance mode and thermoelectric heating mode. When transient operation is considered, even more operational modes become possible. To date, research and applications have primarily focused on thermoelectric generator mode and thermoelectric cooler mode, which have been systematically investigated. However, studies on other operational modes remain relatively limited. Therefore, based on a thermal engineering considerations, this review systematically summarizes the operating characteristics, performance evaluation parameters, and common applications of various operational modes in both steady-state and transient conditions. Additionally, it outlines the evolution of efficiency models for thermoelectric devices under both steady-state and transient operating modes.