Since the first demonstration of efficient organic light emitting diodes (OLEDs) by Tang and Van Slyke in 1987, vast efforts have been devoted to this new technology. However, it was hard to break the internal quantum efficiency limitation of 25% by the use of fluorescent emitters until the first observation of electrophosphorescence from PtOEP by Baldo et al. in 1998, which shed light on a new class of materials based on organometallic complexes. OLEDs utilizing these phosphorescent emitters have the possibility of achieving 100% internal quantum efficiency. Meanwhile, reasonable design of device structures is vital to realize unity IQE. In this review, we will give a comprehensive introduction on the fundamentals of phosphorescence and their application in OLEDs. We will mainly focus on the relation between OLEDs performance and the unique properties of the organometallic complexes used in them, which is essential to realize high performance OLED. We will also give introduction on how to incorporate organometallic complexes into white OLEDs. Finally, we will forecast the development trends in organometallic complexes and OLEDs.