Not only do most people in the most polluted regions of the world live in urban areas, but they also spend nearly 90% of their time inside buildings, a further few per cent in vehicles and only around 6% outdoors, so in this Chapter we shall look briefly at special features of indoor air quality. Despite its importance and complexity, the indoor environment has been much less thoroughly studied than that outdoors. The general perception is that one is ‘safe’ from air pollutants indoors, and many who are today beware of high NO2 concentrations near roads will not worry about cooking with a gas hob that potentially gives them an even higher NO2 exposure. The sources of air pollution that have been discussed so far have been characterised by their release into the atmosphere at large. In general, the concentrations of these pollutants inside buildings will be lower than the concentration outside, because the pollutants will be deposited to internal surfaces during the residence time of air in the building. However, there are many additional sources inside buildings, and the same reduced air turnover that protects from external pollutants may cause the concentrations from internal sources to rise to unacceptable levels. Furthermore, the species involved include not only the same gases and particles that penetrate in from the outside, but a whole range of new pollutants that only increase to significant concentrations in the confines of the indoor environment. The sources include people and domestic animals, tobacco smoke, cooking (especially wood-burning stoves), heating and lighting, building and decorating materials, aerosol sprays, micro-organisms, moulds and fungi. The concentration of any particular pollutant at any time will be the resultant of the external and internal source strengths, deposition velocities to different surfaces, air turbulence and building ventilation rate.