Over the past 15 years, a large amount of effort has been devoted to the development of sampling and analytical techniques to measure the gaseous components of vehicle exhaust. Development of these techniques has been a long, time-consuming process, requiring definition of all factors that can affect accuracy, reproducibility, and the meaningfulness of the test results. In spite of all this activity, further changes and improvements in the test procedures are still being implemented. In contrast, the amount of activity spent in developing methods for determining the quantity and composition of particulate matter in vehicle exhaust has been quite small. There is, at present, no clear definition of what is considered particulate matter in vehicle exhaust. Further, there is no procedure for determining the total particulate emission rate from cars, or a defined test cycle for rating of cars. However, particulate emission control standards are being considered for 1975. The main reason for this lack of definition and procedure is the fact that sampling and measurement of exhaust particulate matter is a very complex problem, and systems to do this have only recently been developed.