important factors that determine the catalytic activity of supported gold catalysts in CO oxidation reaction, with an emphasis on those that provide some information on the underlying chemistry. The criteria that must be met before high catalytic activity can be obtained, and the extreme sensitivity of the final catalyst to all stages of its preparation and treatment before use, are summarized. The influence of the preparation method, the size of the gold particle, the nature of the support and the effect of chloride on the catalysts performance as well as a conclusive analysis of the mechanism of CO oxidation over supported gold catalysts were presented.