The discovery of exceptionally high catalytic activities of small Au particles has initiated an intense research activity to understand its origin. In spite of a large volume of work and converging opinion for many issues, the system is far from being fully understood, and there are conflicting reports of claims of activities and interpretations of remaining issues. This paper focuses one four of the unresolved issues in Au-catalyzed CO oxidation: (1) the importance of the nature of support on catalyst activity; (2) the Au oxidation state necessary for high activity; (3) the sensitivity of the activity to the moisture level in the reaction feed; and (4) reasons for the high activity in small Au particle size and for the strong dependence on particle size and specific morphology. The current understanding of these issues based on available experimental evidence and computational investigations will be summarized and discussed, and we will make an attempt to paint a coherent picture based on a model of the active sites and the reaction mechanism. Aspects that remain unresolved will also be presented.

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