Tobacco use, particularly active cigarette smoking, is a strong modifiable risk factor for periodontitis. Our primary aim is to provide a systematic review of the impact of environmental tobacco smoke on periodontal diseases. Nine publications were identified by electronic literature search up to September 2010. Epidemiological data for a US population sample and a cohort of Japanese men suggest that environmental tobacco smoke exposure may be associated with increase periodontitis prevalence and increased risk of periodontitis progression. Secondly, several methodological issues related to cigarette smoking in clinical and epidemiological research studies are discussed. Tobacco use history is a multi-dimensional phenomenon, with characteristics such as intensity, duration and time since cessation. Understanding these complexities of smoking history and accurate measurement of the exposure, using biomarkers and/or self-reported data as appropriate, are of central importance in clinical and epidemiological studies on the effects of active and passive smoking. The most appropriate approach to assess smoking exposure will vary according to the purpose and design of a particular study.