About a third of the world's population is exposed to secondhand smoke (SHS), despite reductions in smoking prevalence in many countries. Accurate, cost-effective measures of exposure are needed in investigations of the health risks associated with SHS, and in studies of interventions to extend smoke-free environments. There have been important developments in the use of questionnaires, air quality monitoring and biomarkers, but still, there is no single, gold standard assessment of exposure to SHS. Choice of measure depends on circumstances, including cost, scale and time window.