Physicochemical characteristics of Cu-ZSM-5 relevant to its abilities to catalyze the decomposition of NO to the elements and to catalyze selective catalytic reduction (SCR) of NO by hydrocarbons are reviewed. An emphasis is placed on in situ characterization of the catalyst under bona-fide catalytic conditions by X-ray absorption methods. Copper chemistry has central importance. Critical review of literature results demonstrates that Bronsted acidity of the zeolite is not a necessary factor in NO SCR. A model rationalizing ’excessive’ copper ion exchange levels suggests that the exchanging species are not Cu2+ cations, as assumed formally, but rather [CuOH](+) and its related oligomers. The working catalyst exhibits dynamic Cu(II) --> Cu(I) conversions under catalytic conditions. Cu(I) can be the predominant oxidation state under SCR conditions and viable reaction mechanisms will have to take this into account. Reaction pathways for NO decomposition and NO SCR are similar but, in all likelihood, somewhat different. A brief resume is provided concerning adsorbate chemistry which may be important in the mechanisms of both the decomposition and the SCR reactions.