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 impacts on human health (nitrogen dioxide, carbon monoxide, sulfur 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. (Contains 2 figures and 1 table.)