This paper will trace the development of modern regulation of emissions, both local and global, from motor vehicles. To illuminate the principal themes of this story the focus will be on the experiences of the United States and Europe. Among those themes, three stand out, questions that sooner or later must be considered in the development of any environmental policy. First, the theme of federalism. In every country, governments are constituted at various levels of aggregation, from local to national. Which level of government is the most suitable for attacking a given public problem. If different levels of government can fairly claim to have a role in addressing the problem, how will the various responsibilities be assigned and coordinated? In order to develop an effective and efficient public policy, the governments must have both the right incentives and the capacity to do so. Finding the right level of government to address an environmental problem is a tradeoff between two competing considerations. The government’s jurisdiction must be large enough to “internalize the externalities,” as an economist would say. That is, if either the environmental evil or the policy remedy has effects that extend beyond its borders, then the policy-maker’s incentives will very likely be inappropriate. For example, policies to control emissions of stationary-source air pollutants may not be stringent enough if most of the effects of pollution are experienced in neighboring jurisdictions. At the same time, the level of government must be appropriate to the problem. Smaller, more local units of government are more likely to know the preferences of their citizens, yet less likely to have the expertise and experience to deal effectively with particular problems. The second pervasive theme here is the choice of policy instrument: the specific mechanisms used to achieve the environmental objective. It is common to pose two polar types: direct regulation and economic incentives (EI). Rather than commands or requirements, EI instruments provide penalties or rewards to encourage behavior that will improve environmental quality. Another way of putting the difference is this: With direct regulation, there is a bright line that determines whether behavior will be tolerated. With EI, the relationship between performance and consequences is continuous and gradual. There is no bright line, just steadily increasing rewards for better performance.