The article reviews the current status of gas turbine units (GTUs), their thermodynamic basics, design features and the main options for technological growth. Also, such key parameters of the Brayton—Joule cycle that determine the energy efficiency of the gas turbines and combined-cycle gas turbines, including the pressure ratio, the turbine inlet temperature, and the possibility of heat recovery from exhaust gases have been analyzed. It shows the part of modern materials, cooling systems and low-emission combustion chambers to increase the efficiency and reduce the environmental stress.