In optical communication systems, intensity modulation with direct detection (IM-DD) scheme has been used for a long time. However, as it became challenging to achieve high-speed transmission with this method alone, phase modulation was introduced. Early phase-modulated transmission systems used differential encoding and delay interferometers to convert phase-modulated signals into intensity-modulated signals for detection, but this approach imposed limitations on the types of modulation formats that could be used. To overcome these limitations and introduce more advanced modulation formats such as quadrature amplitude modulation (QAM) for increasing a spectral efficiency, the adoption of coherent detection became necessary. This chapter explains these modulation formats employed in long-haul transmission systems.