Atmospheric aerosols are not chemical inert airborne particles whose formation mechanisms, residence time or function in the atmosphere just dependent from mechanical or physical processes. In the course of the aerosol formation process as well as during their stay and transport in the atmosphere chemical reactions play an essential role. Chemical reactions take place at the surface and within aerosol particles and can not only influence gas phase chemistry, e.g. by serving as a reaction media for otherwise slow gas phase reactions, they can also alter the physico-chemical properties of the particles, e.g. their function as cloud condensation nuclei. This review summaries the current knowledge about the chemistry of aerosol, focusing on secondary aerosol formation processes and heterogeneous chemistry of sea salt, dust and soot particles.