Free-electron lasers (FELs) have long been thought to offer the potential of high average power operation. That potential exists because of several unique properties of FELs, such as the removal of `waste heat' at the velocity of light, the `laser medium' (the electron beam) is impervious to damage by very high optical intensities, and the technology of generating very high average power relativistic electron beams. In particular, if one can build a laser with a power extraction efficiency (eta) which is driven by an electron beam of average power PEB, one expects a laser output power of PL equals (eta) PEB. One approach to FEL devices with large values of (eta) (in excess of 10%) is to use a `tapered' (or nonuniform) wiggler. This approach was followed at several laboratories during the FEL development program for the Strategic Defense Initiative (SDI) project. In this paper, we review some concepts and technical requirements for high-power tapered-wiggler FELs driven by radio- frequency linear accelerators (rf-linacs) which were developed during the SDI project. Contributions from three quite different technologies--rf-accelerators, optics, and magnets--are needed to construct and operate an FEL oscillator. The particular requirements on these technologies for a high-power FEL were far beyond the state of the art in those areas when the SDI project started, so significant advances had to be made before a working device could be constructed. Many of those requirements were not clearly understood when the project started, but were developed during the course of the experimental and theoretical research for the project. This information can be useful in planning future high-power FEL projects.