The £rst experiments on FELs, performed in the United States in 1976 (Chapter 9), revealed that the realized FEL designs do not completely meet the demands of their customers, that is, the developers of the Star Wars systems (Chapter 1). This had something to do with key FEL properties such as large ampli£cation coef£cients for the one-signal passage through the interaction area, a moderate longitudinal size, and large mean and pulsed power. As is mentioned in Chapter 1, a known scienti£c-political excitement over FEL developed in full blast during these historical times. Hence, the reaction of the managers of the corresponding programs was the same in the USSR as well as in the United States, that is, to £nd a new, more perfect FEL design scheme. They planned to not only improve the basic properties of the already in-plan systems but also give them principally new directions for their further development. Femtosecond lasers are examples of such new FEL versions. Therefore, historically, the appearance of the £rst designs of the superheterodyne FEL (SFEL) in 1979 to 1980 [1,2] was a completely expected event that adequately ful£lled the aim of the highest management.