The book of the famous American specialist is the first monograph on free electron lasers (LSE) in the world literature. This new rapidly developing field of physics, which arose at the intersection of accelerator physics, quantum electronics and microwave radiophysics, is attracting increasing interest. In the book, the theory of LSE is presented in detail and at the same time at a fairly elementary level, as well as schemes of many experiments are presented. Various types of undulators and electronic accelerators promising for LSE applications are discussed. The characteristics of LSEs operating on high-current accelerators, linear accelerators and storage rings are considered in detail. Projects for creating a microtronic version of LSE are discussed.