Balanced coherent detection technology is widely used in coherent optical communication and microwave photonic systems because the sensitivity can be significantly improved by the technology. Balanced photodetector, as a key device for coherent detection, has been the workhorse of research in recent years. This paper introduces the principle, structure and some parameters of balanced photodetectors and provides an overview on recent advances of the device, with analysis on current problems and development trends. With the development of optical communication technology, the performance of balanced photodetectors will be promoted in terms of speed, responsivity, radio frequency power, and common mode rejection ratio. Moreover, the package module should be highly integrated, with lower cost and lower power consumption in the future.