This research is an experimental investigation into the nature of the interface states in polycrystalline zinc oxide. The role of dopants such as bismuth, praesodymium and barium on the nature of the interface states will be studied. The primary experimental technique to be used is deep level transient spectroscopy using both majority carrier pulse filling and selective optical excitation of interface states. Important new information such as interface density of states is sought along with capture cross sections. Polycrystalline semiconductors are of increasing importance to the semiconductor industry, principally to large area devices. The grain boundary interface states present in polycrystalline zinc oxide are of critical importance to electronic devices known as varistors.