The brochure describes new tunable sources of powerful coherent radiation - free electron lasers. General ideas about the relationship of elementary processes of absorption and radiation of electromagnetic waves with induced absorption and radiation in the active medium, which is an electron beam, are presented. The mechanism of induced radiation in the classical (non-quantum) system, the principle of operation and construction of free electron lasers operating in the mode of a radiation generator and amplifier, the current state of experimental research are considered in detail.