Ever since the first free-electron laser (FEL) is both conceived and demonstrated in the 1970s at Stanford, it has attracted much attention as a high average power light source. Since its light is produced from free electrons, an FEL can be designed to laser at virtually any wavelength. Furthermore, its gain medium cannot be damaged by any conventional means. Accordingly, as there have been several ambitious attempts to design and con.struct high average power FELs over the past several decades. Additionally, an overview of FEL theory is provided,the key techniques and research in the military are summarized, and the future development is prospected.