In the last 10 years an extensive research and development effort has been made to implement the catalytic removal of noxious constituents from automotive exhaust. This effort is finally coming to fruition as this paper is going into print. Vehicles sold in the United States in 1975 will be, in a significant proportion, equipped with catalytic devices. The incorporation of a miniature chemical conversion plant into millions of moving objects operating under widely varying conditions, while being required to assure reliable performance for extended usage periods, has called for the combined skills of scientists, engineers, and designers in several industries. Even so, at present only the catalytic oxidation of carbon monoxide and hydrocarbons has reached the implementation stage. The catalytic removal of nitrogen oxides has turned out to be a still more difficult task. This review will summarize very briefly the present state-of-art of the catalytic devices for the treatment of NO, emissions, but its larger part will be concerned with the scientific background underlying the operation of the devices themselves.