Excited charge transfer complexes (Exciplex) formed between donor and acceptor materials are frequently encountered in organic photonic devices such as in organic light emitting diodes and organic photovoltaics. Formation of exciplexes can be easily identified by the observation of the red shifted emission from those of the component molecules. Generally the PL efficiency of the exciplexes is low so that OLEDs are designed not to form exciplexes at the organic/organic junctions. Formation of exciplexes at the D/A junction is also to be avoided in OPVs since it reduces the dissociation probability of geminate electron-hole pairs formed at the interface. In this presentation we will firstly discuss on the nature of exciplex including the electronic structure, emission processes and diffusion. Further discussion will be given to the application of exciplex forming systems as the triplet harvesting fluorescent molecular system and as the co-host for phosphorescent and fluorescent dopants for ultimate efficiency in OLEDs.