Abstract: Optoelectronic oscillators (OEOs) generate microwave signals with ultra-low phase noise through selfoscillation, which have both optical and electrical outputs. As one of the highest-performance microwave sources, the OEO is expected to be a promising solution to improve the key performance of existing RF systems such as radars and electronic warfare systems. This paper introduces the basic structure and the theoretical models of the OEO, and overviews the research progresses to improve the key characteristics of the OEO, including phase noise, sidemode suppression ratio, frequency range, stability, and size. Studies to extend the functions of the OEO are reviewed, and its applications, especially in radar systems, are discussed.