Three types of systems in which the excitation of oscillations due to high-frequency energy sources is possible have been considered. Despite widely held ideas, one can classify such systems as self-oscillatory. Systems with times of interaction with the energy source that are short in comparison with the period of the oscillations which arise are the first type. Systems of the second type are those having two degrees of freedom, one of which is high-frequency and the other is low-frequency. Transfer of the energy of the high-frequency source to the energy of low-frequency oscillations is achieved by the formation of combination frequencies. Thermomechanical systems are the third type. The role of the high-frequency energy source is to maintain the necessary temperature of the element being heated. Self-modulation of the parameters is the cause of oscillation excitation in such systems.