Background Heat waves will occur in Rotterdam with greater frequency in the future. Those affected most will be the elderly – a group that is growing in size. In the light of the Paris heat wave of August 2003 and the one in Rotterdam in July 2006, mortality rates among the elderly in particular are likely to rise in the summer. Method The aim of the Hotterdam research project was to gain a better understanding of urban heat. Heat was measured and the surface energy balance modelled from that perspective. Social and physical features of the city were identified in detail with the help of satellite images, GIS and 3D models. The links between urban heat/surface energy balance and the social/physical features of Rotterdam were determined on the basis of multivariable regression analysis. The decisive features of the heat problem were then clustered and illustrated on a social and a physical heat map. Results The research project produced two heat maps, an atlas of underlying data and a set of adaptation measures which, when combined, will make the city of Rotterdam and its inhabitants more aware and less vulnerable to heat waverelated health effects. Conclusion In different ways, the pre-war districts of the city (North, South, and West) are warmer and more vulnerable than are other areas of Rotterdam. The temperature readings that were carried out confirm these findings as far as outdoor temperatures are concerned. Indoor temperatures vary widely. Homes seem to have their own dynamics, in which the house’s age plays a role. The above-average mortality of those aged 75 and over during the July 2006 heat wave in Rotterdam can be easily explained on the basis of a) the concentration of people in this age group, b) the age of the homes they live in, and c) the sum of sensible heat and ground heat flux. A varying mix of impervious surfaces, surface water, foliage, building envelopes and shade make one area or district warmer than another. Adaptation measures are in the hands of residents, home owners and the local council alike, and relate to changing behaviour, physical measures for homes, and urban design respectively.