A system combining suitable catalyst technology with a specifically engineered fuelling strategy can lead to improved emission control in lean burn engine applications. The basis of the catalyst design has been to maximise response to rich transients and sustain NOx reduction on reverting to lean conditions. The fuelling control strategy is engineered to optimise the degree and duration of enrichment during accelerations. As an example of the level of NOx control which can be achieved, a concept catalyst has, when used in conjunction with the control strategy, been shown to provide a 29% reduction in NOx emissions over an ECE-15 test, compared with an uncontrolled vehicle. Hydrocarbon conversion is high, impact on fuel economy is minimal and initial throttle response is improved.