Copper ion-exchanged zeolites have widely been studied as the catalysts for NOx emission control. Cu-MFI zeolites, especially over-exchanged Cu-MFI, are very active for the catalytic decomposition of NO. The reaction mechanisms and the active sites suggested are summarized. The newly developed selective catalytic reduction of NO with hydrocarbons in the presence of excess oxygen is then introduced. The role of oxygen and the deactivation of the zeolite catalysts are discussed. In the last chapter, the high ability of Cu-zeolites for NO adsorption is overviewed. Here, some fundamental aspects of the bonding and the molecular diffusion of NO or NO2 Molecules in zeolites are also summarized.