Optical coherent systems, which employ the interference of a received optical signal with a local laser to achieve high signal-to-electrical-noise ratio and capture the magnitude, phase, and polarization of the signal, have grown dramatically in the last decade due to improvements in digital signal processors. The optics of both the transmitter and receiver have grown in complexity, and its deployment has benefitted greatly from the integration offered by silicon photonics. Today’s coherent systems range from 1.2-Tb/s fiber-optic communication transceivers to optical coherence tomography medical instruments. This paper will give an overview of optical coherent systems, present the state-of-the-art in silicon photonic components shipping today, and discuss where silicon photonics is going in this field.