Recent advances in the field of p-i-n type organic light emitting diodes (OLEDs) are reviewed. OLEDs are energy efficient light sources, which will be used in the near future in commercial display and lighting applications. In particular, p-i-n type OLEDs consisting of doped charge transport layers and intrinsic emission layers have been very successful in reducing the operational voltages and increasing the power efficiency. After shortly introducing the p-i-n concept, the advantages of this concept in terms of low operating voltages, high power efficiency and long lifetime are described. In particular, the latest reports on monochrome, white, and top-emission p-i-n OLEDs are summarized.