Recently, a great deal of progress has been made in the area of optical amplifiers and many experiments using them have been conducted aiming at practical applications to various transmission systems. This paper reviews recent developments in optical amplifiers, focusing on Er-doped fiber amplifiers (EDFAs), and their prospective applications. Included are introductory amplification theory to clarify the features of optical amplifiers, comparison of optical amplifiers focusing on EDFAs and semiconductor laser amplifiers (SLAs), and recent typical system applications using EDFAs, including IM/DD transmission, analog transmission, coherent transmission, FDM transmission and other applications.