In recent years, emissions from transportation engines have been studied widely because of the contribution of such engines to atmospheric pollution. During this period the amounts of pollutants emitted, the mechanism of their formation, and means of controlling emissions have been investigated in industrial and government laboratories, as well as at universities. The results of these investigations have generally been published as individual articles in journals, transactions, meeting proceedings, and, frequently, in company reports. This proliferation of technical information makes it difficult for workers in the field to keep abreast of all developments. For this reason, the editors felt the need for a book which would survey the existing state of knowledge in wide, albeit selected areas, and would provide a guide to the relevant literature. This book is intended to fulfill this function. It is recognized that all aspects of transportation engine emissions cannot be explored in a single volume. In this book attention is focused primarily on sources and mechanisms of emission formation within the combustion process, and on measurement techniques. Beyond this objective, no re- strictions were placed on the authors. Within the framework of the general theme each author has been free to treat his subject as he saw fit. The editors have not strived to replace by uniformity the highly personal and attractive divergences of style. Considerable efforts were made, however, to ensure clarity and minimum overlap between the chapters.