An increasing number of countries, including mainland China, are considering enforcing emission regulations for in-use diesel vehicles via remote sensing, a promising technology that measures exhaust emissions from a large number of on-road vehicles without interrupting traffic. To improve the utility of using remote sensing for that purpose, this briefing first explains the current lack of accuracy in estimating tailpipe pollutant concentrations in diesel vehicles, notably for nitrogen oxides (NOx), and then discusses ways to overcome it.