Efficiency droop of nitride-based light emitting diodes (LED) was intensively studied for high-power applications. The origin of droops was attributed to many root causes, for example, carrier leakage, internal piezo-polarization field, Auger recombination, carrier delocalization, and thermal effects. The physical origins were modeled and visualized by various techniques, which will be described in detail in this chapter. Many droop alleviation strategies were proposed by an epitaxial layer structure optimization and device structure innovation, where many of them were widely applied in current commercial products.