This report highlights a problem currently being experienced by new Euro IV and V heavy-duty trucks and buses: Despite meeting more stringent regulatory standards for exhaust emissions during type approval, many vehicles equipped with selective catalytic reduction (SCR) systems have significantly elevated emissions of nitrogen oxides (NOx) during in-use driving, particularly when operating in urban traffic. In some cases, actual in-use urban emission levels may be as high as or higher than those from much older vehicles with engines certified to more lenient emission standards.