It is well established that urbanisation has a significant effect on the local climate. The climate of an urban area will differ from that of a nearby rural area. Modifications to the relevant properties of the urban environment will amplify or reduce these differences. This paper briefly summarises the literature and reports on the impact of these changes. A series of international studies has indicated that modifications to the urban environment have a significant effect on the energy use of buildings. Detailed studies of these issues with regard to the UK are now under way and these new studies are briefly discussed. There is much current interest in attempting to ameliorate potential summer overheating in cities, focusing on the consequences of heatwaves. However, the wider picture should also be considered. In summer in the UK, the urban heat island (UHI) will tend to result in an increased cooling load and an increased number of excess deaths due to overheating. But in winter, the UHI will tend to result in reduced heating loads and a reduced number of cold-related excess deaths. The net effects of these impacts must be borne in mind in the consideration of large-scale urban modifications. (C) 2008 Queen's Printer and Controller of HMSO. Published by Elsevier Ltd. All rights reserved.