For those readers who have been involved with diesel and passenger car engine oils for many years, this chapter may not seem to cover all aspects of this broad topic in great detail. However, for those readers who are interested in the topic and who have not lived through the development of engine oil categories, this chapter is meant to convey an understanding of the challenges and the excitement that practitioners are now facing. The goals of this chapter are to give the reader a full understanding of the lubrication of diesel and gasoline engine oils and their present U.S. categories, and then to concentrate on what makes these oils perform so well today. That role is fulfilled by the additives that are put into these oils, and this chapter will describe the function of these remarkable components.