Abstract: Environmental particulate matter levels are closely linked to human health, with road traffic emissions being a significant source of urban particulate matter. As the mass concentration of particulates in vehicle exhaust approaches the measurement lower limit, the urgent need arises to develop more sensitive methods for measuring and evaluating particulate matter. Over the past two decades, motor vehicle particulate emissions have garnered increasing attention, leading to numerous changes in global emissions regulations. These include stricter standards, new indicators, and expansion to broader off-road applications, necessitating new requirements for real-time, highly sensitive, onboard instruments. The measurement of particulate matter number concentration serves as a crucial supplementary approach to evaluating vehicle emissions pollution. This paper reviews the evolution of this metric in relevant regulations and existing methods for measuring particulate number concentration in vehicle exhaust. It provides an overview of the principles and application ranges of both current and newly developed instruments, offering insights into the technical progression of measuring and evaluating particulate number concentration in motor vehicle exhaust emissions.