This is an overview of the main physical ideas and experimental results concerning the application of field effect transistors for the generation and detection of TeraHertz ( THz) radiation. Resonant frequencies of the two-dimensional plasma oscillations in FETs increase with the reduction of the channel dimensions and reach the THz range for sub-micron gate lengths. When the mobility is high enough, the dynamics of a short channel FET is dominated by plasma waves. This may result, on the one hand, in a spontaneous generation of THz frequencies by a dc current and on the other hand, in a resonant response to imaging.incoming THz radiation. In other cases , when plasma oscillations are over damped, or are generated on the gate edges the FET can operate as an efficient broadband THz emitters and detectors and can be used for