The aim of this book is to provide an informative idea in the field of Organic Light Emitting Diode (OLED) on thin film technology. Here I have tried to make the book useful for giving a clear and systematic treatment of the subjects involved in the study of OLED and incorporating the brief review of all direction of possible research which should bring the clear idea of development and advancement of OLED in the field of display technology. This book also contain a brief review of OLED work in INDIA from beginning which would be more useful for the reader to know the continuous development of the OLED research in our country. I hoped that the book in the present form will be able to serve reader a better understanding to the theory, operation and working of OLED including the concept of proper thickness selection of the materials to reproduce OLED for commercial purposes. To complete the book I take a survey of the works on OLED which are mentioned in the reference section. Finally I hoped that this book will be greatly helpful to the readers for better understanding the development of OLED in optoelectronics.