This chapter explains the concern about atmospheric aerosols, which are liquid droplets or particles that are suspended in the atmosphere. They absorb and reflect light, play an important role in heat balance, provide condensation nucleus points, and influence atmospheric chemistry. They can be emitted directly, or formed in the atmosphere to make secondary aerosols. While there are many naturally-occurring aerosols, transport and industrial processes are releasing chemical gases into the atmosphere, and land use and combustion release dust and smoke. Aerosol systems are complex and involve both natural and anthropogenic sources and sinks, and have an effect on the radiative balance. At the moment, the net global effect of aerosols is a cooling effect. Examples are given of regime shifts caused by aerosols.