Engineering of catalytic reactors for gaseous pollutants abatement is presented. The model process is catalytic combustion of volatile organic compounds. Catalytic reactors suitable for environmental applications are presented with an emphasis on structured reactors and layered catalysts. Physical properties of porous catalysts are presented. The catalyst efficiency resulting from chemical reaction coupled with diffusion in porous solids is discussed. Special attention is given to the mass transfer to the catalyst surface and within a porous catalyst body. A mathematical one-dimensional classic model of a tubular catalytic reactor is presented. Permissible model simplifications are precisely discussed with stress on the process temperature. Heat and mass transfer as well as flow resistance characteristics are given for both classic, packed-bed reactors and modern reactor structures, e.g., monoliths, stacked wire gauzes, solid foams. The problem of heat losses is shortly discussed.