Mobile sources of air pollutants largely comprise of technology emissions from modern means of transport. Some may consider this as the insoluble dilemma of human progress at its own peril. Comparisons between the air pollution implications of different transport modes are also of interest. Which comparisons are relevant depends partly on the route length and terrain. Long-distance passenger travel is dominated by aircraft, but most freight is carried on the surface by ships. Trains compete with aircraft on land-based trips of up to several hundred kilometres. Roads and railways compete over shorter distances. Ships are involved at all distances, but need water. Ships also have specialist niches such as ferries and long distance movement of freight and raw materials. The air pollution impact can be quantified in various ways. In order to compare emissions per passenger km (or per tonne km for freight) we also need to know the capacity and the load factor (proportion occupied). The majority of 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. There is also a section describing the development of emissions inventory specifically for mobile sources.