Multi-tone amplification is a novel technique to effectively suppress stimulated Brillouin scattering (SBS) effect in narrow-linewidth fiber amplifiers. The corresponding basic principles are described in detail. Multi-tone amplifications can be classified into large and small wavelength separation amplifications. Important outcomes in theoretical and experimental researches of these two kinds of amplifications are overviewed and their superiorities in SBS suppression are analyzed. It is pointed out that the large wavelength separation amplification shows great advantage in enhancing the output power of the single frequency laser and the small wavelength separation amplification is of great value to further enhance the output power of current high power coherent beam combination system of fiber lasers.