Following two decades of intense research globally, the organic light-emitting diode (OLED) has steadily emerged as the ultimate display technology of choice for the coming decades. Portable active matrix OLED displays have already become prevalent, and even large-sized ultra-high definition 4K TVs are being mass-produced. More exotic applications such as wearable displays have been commercialized recently. With the burgeoning success in displays, researchers are actively bringing the technology forward into the exciting solid-state lighting market. This book presents the knowledge needed for students and researchers from diverse disciplines to understand the underlying principles in OLED technology. It begins with a brief history and fundamental working principles of OLEDs. After introducing the fundamentals, it discusses more efficient OLED designs, as well as advanced strategies to enhance the performance. The text covers in detail important areas such as top-emission, p- and n-type doping, device stability, light extraction, and stacked white OLEDs. It also throws light on the current industry practice and major areas of focus in the near future.