Optoelectronic oscillators (OEOs) can generate high frequency, broadband tunable, and low phase noise radio frequency (RF) signals that can be applied in 5G communication, radar detection, and sensing. The basic structure of an OEO includes lasers, modulators, energy storage media, photodetectors, RF amplifiers, and RF filters. The key to achieve low phase noise is high-Q energy storage media, such as low loss long fibers or high-Q optical resonators. As Dr. Yao Xiaotian proposed OEOs in 1996, various OEO structures have emerged; however, a majority of these are composed of discrete devices, making them bulky, expensive, and inconvenient to use. With the development of various integrated optical material platforms, it has become possible to construct small-sized, low-cost, and more reliable integrated OEOs, such as silicon-on-insulator (SOI), indium phosphide, and chalcogenide OEOs. However, due to high loss, nonlinearity, and temperature sensitivity of these materials, it is difficult to achieve the high-performance electro-optic modulators and high-Q energy storage media required for OEOs. Consequently, thin-film lithium niobate (TFLN) has been widely used for achieving high-performance electro-optic modulators and high-Q microcavities due to its wide transparent window, large linear electro-optic coefficient, and low loss characteristics; thus, it achieves high frequency, broadband tunable, and low phase noise RF signals. This article reviews the development process of OEOs over the past 30 years, from discrete OEOs to integrated OEOs. It also discusses future development prospects of integrated OEOs.