The Organic Light Emitting Diode (OLED) world is one of the most fascinated fields of research with an enormous technological application market. Besides the actual use in displays, the intrinsic and unique properties of that electroluminescent devices opens new potential applications since efficient lighting to decorative environments, with a very simple incorporation in architectural design. This book addresses the development of OLEDs based on rare-earth and transition-metal complexes, focusing in special the Europium, Terbium, Ruthenium and Rhenium. The idea is to explain how these organic materials can be used to build organic-LEDs, from chemistry synthesis to device operation and the related charge transfer and confinement. A relationship between the chemistry molecule 'design', considering the energy levels of the charged and non-charged molecule parts, the ionized metal ions and the carrier injection / bulk transport, will be the basic for the device physical explanations and optimization. Further developments, taking into account the actual state of the art and the expected pathways will be proposed. The book is intent to have intensive experimental results for better explanation it is intent to be a general book for scientists and engineers that work in this new OLED framework, but keeping a didactic utility for graduation students and teachers working on optoelectronics. Such kind of book (at my level of knowledge) does not exist actually (excluding a few review papers). Naturally there are some overlapping areas with others books (and there exists many about OLEDs and PLEDs) but I'm sure that we can keep this overlap field restricted to the introductory areas.