An overview is given of the selective catalytic reduction of NOx by ammonia (NH3-SCR) over metal-exchanged zeolites. The review gives a comprehensive overview of NH3-SCR chemistry, including undesired side-reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe-ZSM-5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft-neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3-SCR reaction over Fe-ZSM-5, with isolated species being the most active.