Considerable energy has been expended over the last two decades in developing methods for identifying the presence of asthma among participants in population studies. The objective of this endeavour has commonly been to enable epidemiological research on risk factors for asthma, with the ultimate aim of preventing the disease. However, there are other reasons for measuring the prevalence of asthma in populations, including the desire to assess the burden of disease attributable to asthma, to track changes over time, and to compare the prevalence among population groups defined by geographical, demographic or social factors. Two major international collaborative studies arising from this work have made major contributions to our knowledge about asthma