The selective catalytic reduction of NO by hydrocarbons in the presence of a large excess of oxygen is one of the greatest challenges facing catalytic chemists today. The first catalyst reported to show substantial promise for this reaction was ion-exchanged Cu/ZSM-5, and this has since become the ’bench-mark’ lean NOx catalyst. A large number of studies have been performed on this catalyst system, and the characteristic features of the reaction over this material are fairly well defined. These characteristic features are collected together and analysed within this review article. In addition, a number of investigators have attempted to determine the reaction mechanism over this catalyst. These studies are discussed in detail, and the evidence both for and against each of the mechanistic proposals is assessed. Mechanistic studies performed on related catalyst systems are also reviewed herein, and are discussed with respect to the work carried out on Cu/ZSM-5.