The authors have reviewed the state of the art of free-electron lasers (FELs) and concluded that FELs based on electrostatic accelerators with energy recovery can fundamentally change the economics of producing coherent radiation over a very broad part of the spectrum. This is particularly true of applications requiring high average power and high efficiency. They illustrate the principles of an FEL system based on a straight-geometry electrostatic accelerator and multistage depressed collector. By using an electron gun in which the current can be pulsed or modulated and a resonator cavity which can be dumped in one radiation transit time, it is also possible to achieve very high-peak-power levels by building up the cavity radiation field with a pulse train of electron-beam particles. Calculated and experimental results are given, showing that operation of an electrostatic accelerator FEL in the optical regime requires improvement in the state of the art of either the maximum accelerator energy or in the minimum wiggler period. A number of present and future uses for high-power FELs are listed.