High-power diode lasers with excellent beam quality are used in various applications in laser processing, laser communication, scientific research, and other fields. Improving the output power and beam quality of diode lasers remains a primary focus of international research and a hot topic in the field. Among numerous strategies to augment diode laser output power, beam combining technology stands out as the simplest and most effective. Incoherent beam combination often increases output power at the expense of decreasing spatial, polarization, or spectral characteristics, making it suitable for applications with less stringent beam requirements. Conversely, coherent beam combination not only increases the output power of diode lasers but also improves the beam quality and narrows the spectral linewidth, presenting an important direction for advancing the development of high-brightness, narrow linewidth diode laser technology. This article concisely delineates the principles and requirements of coherent beam combinations. Starting with phase locking technology, this paper presents a comprehensive review of recent developments in coherent beam combining technology for diode lasers. The advantages and drawbacks of active and passive phase locking are summarized. Active phase locking technology, which employs a master oscillator power amplifier structure and implements a phase-negative feedback, has advantage in the number of combined units. This enables high-power coherent output, eventhough the technology has a structurally complex nature. Meanwhile, passive phase locking technology, characterized by a simple structure, typically achieves phase locking among units through diffraction effects in an external or a common cavity, demonstrating its self-organizing phase locking characteristics; however, the technology is less effective in realizing high-power output. Finally, the future development of coherent beam combining technology for diode lasers is discussed.