A summary of the current understanding of the lean NO, reduction reaction on various metal oxides, particularly Cu-ZrO2, is presented. Whenever appropriate, a comparison with the behavior of the corresponding ZSM-5-based catalysts is made. It is concluded that there are many mechanistic similarities between metal oxide-based and zeolite-based catalysts. Thus, the catalytic properties are determined more by the nature of the active centers than by the nature of the support. Potential advantages offered by metal oxides should make them attractive candidates for practical applications.