In the photoluminescence (PL) spectra of ZnO, typically there are emission bands in the UV and visible (green, yellow, blue and violet) regions. The UV emission is usually considered as the characteristic emission of ZnO, and attributed to the band edge transition or the exciton combination. Although the emissions in the visible regions are universally considered to be associated with the intrinsic or extrinsic defect in ZnO, extensive controversies have existed for more than two decades on the clear defect centers, and unambiguous electron transitions are not yet known in detail. For green emission, a number of different hypotheses have proposed, such as transition between singly ionized oxygen vacancies and photoexcited holes, transition between electrons close to the conductive band and deeply trapped holes at Vo++, surface defect, etc. the yellow-orange emission have been assigned to interstitial oxygen and dislocation, as well as Li dopants.