Multilayer organic light emitting devices were studied with an aim of achieving highly efficient emission. It was shown that organic fluorescent materials had advantages in achieving very high efficiency of outer emission, more than 5 lm/W, and in blue-emitting materials. Three layer devices which had double hetero-junctions between two different organic layers were studied with an objective of accomplishing confinements of injected carriers and generated excitons within the emission layer. For the former purpose, requirement for the relation between the electronic energies in the carrier transport layer and those in the emission layer was clarified. The requirement for confinement of the excitons was also demonstrated from the excitation energy point of view. The emphasis was laid on the size effects in three layer EL devices. Variation of the fluorescent lifetime or the radiative decay rate with the spacing between the emission layer and the metallic electrode was discussed theoretically and experimentally. It was demonstrated that enhancement and suppression of the spontaneous emission from the excitons occured periodically with variateion of the spacing. Variations of EL intensity and pattern of outer emission through semitransparent glass substrate with the spacing were discussed also.