This paper reports the results of Phase I of a study of heavy-duty diesel inspection and maintenance (I&M). Heavy-duty diesel vehicles have generally been exempted from I&M programs, due to the lack of a suitable inspection procedure and uncertainty as to the reduction in excess emissions which might result. This paper describes common types of tampering and malmaintenance which can lead to excess emissions from heavy-duty diesel trucks and buses, and presents the results of two surveys aimed at estimating the frequency of occurrence of these problems. Data showing the effects of some of these problems on engine emissions are also presented. Finally, a preliminary estimate of the impact of excess emissions from diesel trucks and -buses in California is presented. Further work to develop suitable inspection procedures and to evaluate the costs and benefits of alternative I&M programs is being performed in Phases II and III of the project.