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.