The effect on health of urbanisation is two-edged. On the one hand, there are the benefits of ready access to healthcare, sanitation, and secure nutrition, whilst on the other there are the evils of overcrowding, pollution, social deprivation, crime, and stress-related illness. In less developed countries, urbanisation also opens the door to ‘western’ diseases, including hypertension, heart disease, obesity, diabetes and asthma. Here we review some of the health-related aspects of urbanisation, and comment on strategies designed to improve urban health. Because there is such a clear divide between the long process of urbanisation in industrialised western nations and the relatively recent explosive expansion in resource-poor countries, they are discussed separately.