The ecology of cities assesses the health and well-being benefits of urban greenspace, vegetated areas and water bodies. Ecology in cities examines how the characteristics of the urban landscape mosaic, and various parts of it, affect human health and well-being. Human consumption and behaviour in one urban area can affect the health and well-being of people in other, such as through export of waste and trans-boundary emissions and pollutant flows. Environmental, economic, technological, social and individual risk factors and impacts affect urban populations at four urban ecosystem scales: patches within the urban mosaic, built-up areas, urban regions and the global scale. Increasing urban built-up area sizes lead to more severe heat island effects, particularly for people with inadequate cooling living in buildings that retain heat. Urban people benefit in varied ways from everyday contact with nature, but some urban greeenspaces create both health benefits and health hazards. Cultural or social contrasts in responses to urban nature and greenspaces vary. Despite a general consensus that a healthy urban ecosystem can improve human health and well-being, a large part of the global urban population in low latitude, low income cities, the poorest urban dwellers are likely to suffer from contact with many aspects of nature. © 2012 Elsevier B.V.