Due to the tremendous increase in communication traffic in recent years, more and more efforts in research have been directed towards developing highly efficient broad-band fibre amplifiers that will fully exploit the low-loss band of silica fibres in order to increase the transmission capacity of wavelength-division multiplexing (WDM) networks. These broad-band amplifiers must be able to amplify the new short wavelength band (S-band) in addition to the existing C- and L-bands. Thulium-doped fibre amplifiers (TDFAs) are a promising candidate for the S-band amplification. This book reviews TDFA's structure and its amplification mechanism.