In this short topical review, a brief account of the evolution of defects due to controlled changes in polycrystalline zinc oxide has been presented. X-ray diffraction, Positron annihilation spectroscopy and optical absorption spectroscopy has been employed to understand various defective states of ZnO. Thermogravimetric analysis, room temperature resistivity and photoluminescence measurements (just mentioned) have been used to throw more light on this topic. A coherent scenario in the light of previous works in this field has been discussed. At the end discussion on the magnetic studies on ZnO-based systems has been added in short. (C) 2008 Elsevier Ltd. All rights reserved.