This chapter provides a broader understanding of the road traffic emissions of air pollutants. This is followed by a relatively brief description of other non-road mobile machinery sources, rail transport, shipping and aircraft emissions. A small fraction of the exhaust gases is directed back into the air inlet. The chapter discusses road transport and emissions from planes, boats and trains. It explores the sources of motor vehicle emissions as a group, because the pollutants are emitted together. High urban population density presents myriad challenges that include servicing requirements for transportation, both of passengers and freight. More than 50% of the global population lives in urban areas and by 2030, the number of urbanites is expected to reach almost 5 billion. Catalytic conversion of the main pollutant gases in the exhaust has been used in America and Japan for many years and as the arrival of Euro 1 standards has been mandatory in the countries of the European Union.