A method and apparatus for heating an exhaust gas treatment system comprising a catalytic converter and a heat source, the catalytic converter having a first temperature above which the catalytic reaction is of high efficiency, whereby the heat source is controlled so as to generate a heated sub-volume of the catalyst which is in excess of the first temperature and propagates along the length of the catalytic converter such that at any one time the total conversion efficiency of catalytic converter is sufficient to remove a target amount of pollutants from the exhaust gas stream.