Despite an estimated one billion people around the world living in slums, most surveys of health and well-being do not distinguish between slum and non-slum urban residents. Identifying people who live in slums is important for research purposes and also to enable policymakers, programme managers, donors and non-governmental organisations to better target investments and services to areas of greatest deprivation. However, there is no consensus on what a slum is let alone how slums can be distinguished from non-slum urban precincts. Nor has attention been given to a more fine-grained classification of urban spaces that might go beyond a simple slum/non-slum dichotomy. The purpose of this paper is to provide a conceptual framework to help tackle the related issues of slum definition and classification of the urban landscape. We discuss: The concept of space as an epidemiological variable that results in 'neighbourhood effects'. The problems of slum area definition when there is no 'gold standard'. A long-list of variables from which a selection must be made in defining or classifying urban slum spaces. Methods to combine any set of identified variables in an operational slum area definition. Two basic approaches to spatial slum area definitions - top-down (starting with a predefined area which is then classified according to features present in that area) and bottom-up (defining the areal unit based on its features). Different requirements of a slum area definition according to its intended use. Implications for research and future development.