There has been a great deal of interest in developing new materials for the fabrication of light emitting diodes, built from molecular and polymeric materials. A significant motivation for this work has been their potential for use in future flat panel displays. Our work has been focused on developing new emissive and charge transporting materials for these devices. Our materials have led to marked improvements in the efficiencies of these devices. In particular, we have efficiencies for LEDs close to 100%, by using phosphorescence based emitters. The phosphorescent dopants in these devices are heavy metal containing complexes (i.e. Pt, and Ir compounds). I will discuss the basic mechanism of electroluminescence in OLEDs, and then elaborate on the use of phosphorescent complexes to achieve high EL efficiencies in monochromatic OLEDs. In the discussion of electrophosphorescence I will highlight specifically how we tune emission color in both Pt and Ir based emitters by careful design of both the metal complexes and ligands. We have also demonstrated white light emitting OLEDs, using many of the same emissive materials. These devices emit simultaneously from monomer and dimer/aggregate states of Ir and Pt dopants. The result is an emission spectrum that covers the entire visible spectrum, giving true white illumination. We have recently prepared binuclear Pt complexes and can show that the low energy emissive species in the white OLEDs is most likely a ground state dimmer or aggregate. This aggregate state undergoes a structural change in the excited state, similar to an excimer. I will discuss the mechanism of electroluminescence in this system and describe our recent advances in achieving high efficiency white electroluminescence.