Atmospheric chemistry is a relatively new scientific discipline. Research in this area has intensified dramatically in the last 3-4 decades because of the concerns that growing emissions of pollutants could have severe consequences for humankind. Although the major chemical compounds present in the atmosphere were identified long ago, it is only during the second half of the 20th century that the complexity of chemical processes was highlighted and carefully investigated. Laboratory studies as well as field campaigns and modelling studies have helped elucidate the mechanisms that, for example, control the oxidizing power of the atmosphere or the formation and fate of aerosol particles. In recent years, research has emphasized the role of atmospheric chemistry in the global environment, and specifically the impact of global air pollution on climate change. The important issue of air quality, initially considered as a local or regional problem, is now regarded as a global problem. Much work has been done to assess the causes and consequences of air pollution (including health effects) in urban areas, especially since it was shown by Haagen-Smit (1952) that the high levels of ozone recorded in Los Angeles were due to the emissions of anthropogenic hydrocarbons and nitrogen oxides. Since the early 1970s, it has been known (Crutzen, 1973) that the same type of chemical processes operates even far away from highly industrialized regions, and that the entire atmosphere must be regarded as a complex chemical reactor. This is particularly true in the tropics, where the intense solar radiation and the large natural and anthropogenic sources for many reactive compounds are responsible for high photochemical activity that determines to a large extent the global chemical properties of the atmosphere.