Extensive effort has been given to identifying and quantitating the organic genotoxins associated with airborne particles; however, very little is known about the mutagenicity of vapors and gases, especially those associated with ambient air samples. This overview of the literature and recent U.S. Environmental Protection Agency efforts examines some of the critical aspects associated with the bacterial mutagenicity testing of gases and vapors. For example, the sensitivity of the test system depends upon the method of exposure (preincubation, gases over plates, liquid suspension, etc.); and the method-of-choice appears to depend upon vapor pressure and the solubility of the compound under investigation. Also, the selection of test chamber and other materials is very important since plastic petri dishes have been shown to absorb up to 85% of any haloalkane vapors within an exposure chamber. Investigations have also shown the importance of exposure rates, method of gas mixing, role of metabolic activation systems, the occurance of photoactivation. In ambient studies, the identification and quantitation of volatile organic mutagens is quite difficult; however, due to their abundance these efforts are very important.