The most important systems for downstream reduction of NOx under lean conditions of exhaust gases are discussed. The systems operate either continuously under direct chemical reduction of NOx to nitrogen, or discontinuously with prior adsorption. The selective catalytic reduction (SCR) of nitrogen oxides using ammonia (processed in an NH3 generator) or ammonia-forming substances (urea as solution or solid) as reducing agents is almost at the application stage in trucks, while hydrocarbons as reducing agents (so-called HC-SCR) are not sufficiently selective for NOx reduction because the hydrocarbons are themselves predominantly oxidized. The principle of NOx storage as nitrates followed by regeneration and reduction to nitrogen requires no separate or external reducing agents and claims therefore to meet future emission standards as long as problematic sulfur incorporation is avoided. The common characteristics of all these NOx abatement systems is that they are all based on the principle "oxidation before reduction" and involve the key molecule NOx.