With the increasing traffic of long-distance communication, the need for high-speed communication with gigabit-per-second order has risen, driving the demand for innovations in transmission technology. The issues associated with increasing the bit rate in optical regeneration repeater systems became more apparent, leading to increased interest in optical amplification, which had been under investigation for some time. This chapter provides an overview of optical amplifiers and transmission systems using them in the repeaters. The first commercial Trans-Pacific submarine cable systems that adopted optical amplifier is also introduced.