The light duty diesel vehicle has undergone a dramatic improvement in customer acceptability and market penetration in Europe in recent years, largely because of significant improvements in engine design. The diesel engine is an inherently low emissions technology and until recently, it has not required secondary emission control technology such as catalytic converters. However in the near future more stringent legislation in the EC and USA poses some major challenges for the diesel passenger car which will be difficult to meet without secondary systems. The primary problem is control of NOx under lean burn conditions but this must be considered in the context of the other requirements imposed on a modern diesel car. Useful lean NOx control options must satisfy these other requirements as well as exhibiting lean NOx performance. Without successful lean NOx catalysts the diesel car will struggle to make further advances in the market.