This article describes the principles, applications and performances of methods to measure gas-phase air pollutants that either utilise passive or active sampling with subsequent laboratory analysis, or involve automated in-situ sampling and analysis. It focuses on air pollutants that have adverse impact upon human health (nitrogen dioxide, carbon monoxide, sulphur dioxide and benzene), vegetation (ozone) or climate change (ozone, carbon dioxide, methane) and nitrous oxide). It begins with an explanation of why air pollutants are measured, and concludes with prospects for the future, and an illustration of recent trends in air pollutants derived from road traffic recorded in central London
