AbstractThis essay explores the medieval belief that disease was spread by the foul odours arising from such common nuisances as butchers’ waste, dung heaps, stagnant water, and tanneries, as a result of which great importance was placed upon fresh, clean air in medieval cities, especially in times of pestilence. It examines the medical rationale behind these ideas, which derived from Classical Greece and reflect sophisticated assumptions about human physiology. It also considers the numerous sanitary measures that were introduced at both a national and local level in pursuit of a hygienic urban environment, and the extent to which ordinary citizens recognised the vital role played by the air that they breathed in preserving or endangering communal health.