Coherent optical receivers have initially received significant research interest in the 1980s. The first commercial optical transmission systems were implemented using intensity-modulation and direct-detection (IM-DD). The use of coherent detection and digital signal processing (DSP) enable optical system that fully leverage the benefits of advanced modulation formats and provide previously unavailable functionality in systems with direct detection, such as the use of phase and the polarization dimensions as independent orthogonal means to convey information. There are many possible implementations for an optical polarization-multiplexed (PM)-QAM transmitter. This chapter presents the most commonly used transmitter architecture and is compatible to arbitrary QAM constellation generations. Coherent optical transceivers now utilize DSP with the transmitter being responsible for modulation, pulse shaping and pre-equalization, and the receiver responsible for equalization, synchronization, and demodulation. Coherent detection and DSP were the key enabling technologies in the development of 100G optical-transmission systems.