White organic light-emitting diodes (WOLEDs) are now approaching mainstream display markets, and they are also being aggressively investigated for next-generation lighting applications because of their extraordinary characteristics, such as high efficiency, high luminance, lower power consumption, wide viewing angle, fast switching, ultralight weight, and flexibility. In this paper, we first introduce the various approaches to realize WOLEDs, and then summarize the properties and differences of the four types of WOLEDs from the perspective of the emitting materials. The recent development of fluorescent, phosphorescent, fluorescent/ phosphorescent hybrid, and delayed fluorescence WOLEDs is comprehensively illustrated. By combining with our published works, we systematically review the device structures, design strategies, working mechanisms, physical theories, and electroluminescent processes of the reported WOLEDs. Then, the development of flexible WOLED is presented. Finally, the existing problems and trends of WOLEDs are discussed.