Coherent detection can detect both the amplitude and phase of an optical signal, offering higher receiver sensitivity. For this reason, its application to optical communication was actively explored worldwide during the 1980s. However, due to technological limitations at the time, practical implementation was difficult. With the emergence of optical amplifiers, the need for coherent detection diminished, and most research efforts were suspended. After 2000, as demand for higher-capacity optical fiber communication systems increased, the conventional direct detection method became insufficient to meet this demand. This led to renewed interest in optical coherent communication. A new approach, known as digital coherent detection, was proposed to address the challenges of traditional coherent communication through advanced digital signal processing technologies. With the introduction of digital coherent detection, it became possible not only to employ advanced modulation formats that utilize both the amplitude and phase of light, but also to compensate for all of the chromatic dispersion accumulated along the transmission path using DSP at the receiver. As a result, dispersion management in the transmission line became unnecessary, enabling the minimization of fiber loss and nonlinearity. This chapter explains digital coherent detection technologies which have become indispensable in recent high-capacity optical communication systems.