Organic light-emitting diodes (OLEDs), invented by Ching Wang Tang in 1987, were a scientific curiosity initially until Richard Friend's discovery of electroluminescence from polymers. Since then, a lot of active research activities have been undertaken to develop highly luminescent and charge-carrier-transporting organic materials, as well as efficient and stable OLEDs. In the last decade, progress has been made in this active area of research, and many companies like Samsung, Sony, and others began to commercialize OLEDs for flat-panel display applications. Compared to liquid crystal displays (LCDs), OLEDs offer many advantages, such as higher contrast ratio, higher response speed for video, and lower power consumption. Today, next-generation display technology based on OLEDs is a new enabling technology that promises to eventually replace LCD-based displays. Another promising application is in the area of solid-state lighting. In order to achieve this new goal, the performance of OLEDs needs to go to the next level, with higher efficiency and longer lifetime. Further development in high-performance materials such as efficient phosphorescent emitters, stable and high-bandgap host materials, and efficient carrier-transport materials are critical to the development of OLEDs with enhanced efficiency and long-term stability. In addition, further understanding of loss and device degradation mechanism(s) is also very important. In OLEDs, almost 75% of the photons generated are lost, and novel approaches to enhance light extraction are vital for solid-state lighting applications.