Climate change will affect the health of urban populations. It represents a range of environmental hazards and will affect populations where the current burden of climate-sensitive disease is high — such as the urban poor in low- and middle-income countries. Understanding the current impact of weather and climate variability on the health of urban populations is the first step towards assessing future impacts. In this paper, we have reviewed the scientific evidence for the effects of temperature, rainfall and extreme events on human health, in particular the impacts of heat waves and floods. The methods for assessing the risks of climate change are undergoing development, and there is a need to shift the focus from global and regional to local studies. Sectoral approaches to climate change impact assessments often ignore the effects on health. There is a need to better describe the risks to health from extreme weather events as well as improve the effectiveness of public health interventions. Improving the resilience of cities to climate change also requires improvements in the urban infrastructure, but such improvements may not be achieved quickly enough to avoid an increased burden of disease due to global climate change.