Abstract Environmental tobacco smoke (ETS) is formed from cigarettes as sidestream and exhaled mainstream smoke diffuse into ambient air. Detailed studies are reviewed which describe how sidestream smoke is formed, its acceleration away from the cigarette and its chemical properties. As the smoke streams diffuse into the atmosphere they become greatly diluted and physical and chemical changes occur. A quarter of the material in sidestream particles evaporates, so that ETS nicotine is virtually entirely in the vapour phase, and the particles shrink. As cigarettes are smoked, the levels of ETS components rise and then fall exponentially due to air exchange and deposition of smoke particles into surfaces. The decay of ETS also depends on the particular component, with nicotine decaying faster than other substances. In real-world environments, ETS is found along with chemicals and particles from many sources. Studies are reviewed which quantify the contribution of ETS to various indoor air environments. These include determination of the ETS proportion of total respirable particles, measurement of nicotine as a specific ETS marker, and comparisons of chemicalspresent in matched smoking and nonsmoking environments. The ETS contribution of volatile organic compounds in air is much less than that from other sources. The review emphasises the need for tobacco specific analytes to be used as ETS markers and/or to apportion ETS particulate matter from total particulate matter in the atmosphere.