White organic light-emitting diodes (WOLEDs) have drawn significant attention since Kido and co-workers first observed white emission from devices with a single emitting layer (EML), by doping dyes into polymers [1] and [2], and devices with three separated EMLs [3]. During the past two decades, researchers in both academia and industry have invested a great deal of effort in developing WOLEDs for applications in flat-panel displays and solid-state lighting sources. Recent prototypes of WOLEDs have even been fabricated commercially. Benefiting from favorable features, such as homogenous large-area emission, and being lightweight and ultra-thin, WOLEDs are expected to become a complementary or competitive candidate in the display and lighting markets in the near future. In the case of lighting applications, WOLEDs are expected to achieve low energy consumption, long lifetime and low cost to compete with other light sources such as fluorescent tubes and inorganic LEDs. Much more work must be done to improve WOLEDs, which include more emphasis on developing efficient and stable materials as well as improving current device structures. Such improvements should enhance the performance and propel WOLEDs to reach their full potential for commercial applications.