Despite extensive contributions to the literature since 1975 regarding selective catalytic reduction (SCR) catalysis with ammonia for NOx removal from combustion gases, there is still on-going, controversial discussion in academia concerning both catalytically active sites in the SCR mechanism and deactivation phenomena with regard to practical SCR applications. New developments such as SCR of NOx by hydrocarbons are still thwarted by problems relating to the development of suitable materials. Efforts to replace SCR technology by selective catalytic decomposition (SCD) for NO remain confused in both theory and experiment.