The operating voltage of organic light emitting diodes (OLEDs) is important for the power consumption of active or passive matrix displays since it influences both the power consumption of the OLED itself and the power consumption of the driver circuitry. We have shown that very low operating voltages can be achieved in small-molecule OLED by intentional electrical n- and p-type doping. Even more important than the reduction of the voltage is the fact that doping of the charge carrier transport layers improves charge injection, making it basically independent on the actual contact work-ftmctions. Organic light emitting diodes (OLEDs) with electrically doped transport layers show significantly improved properties: For instance, we have achieved a brightness of 100cd/m(2) already at a voltage of 2.55V (based on a simple singlet emitter system), well below previous results for undoped small-molecule devices. With phosphorescent emitter dopants, high quantum and power efficiency of OLEDs with doped transport layers can be achieved: operating voltages and current efficiencies of 3.1V and 44cd/A (corresponding to approx. 44lm/Watt at 100cd/m(2)) are reported here. Inverted and fully transparent devices with parameters comparable to standard bottom-emitting OLED have been demonstrated as well.