Using a combination of depth-resolved cathodoluminescence spectroscopy, electronic transport, and surface science techniques, we have demonstrated the primary role of native defects within ZnO single crystals as well as native defects created by metallization on metal -ZnO Schottky barrier heights and their ideality factors. Native defects and impurities resident within the ZnO depletion region as well as defects extending into the bulk from the intimate metal - ZnO interface contribute to barrier thinning of, carrier hopping across, and tunneling through these Schottky barriers. Chemical reactions at clean ZnO metal interfaces lead to metal-specific eutectic or oxide formation with pronounced transport effects. These results highlight the importance of bulk crystal quality, surface cleaning, metal interaction, and post-metallization annealing for controlling Schottky barriers. (C) 2008 Elsevier B. V. All rights reserved.