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Competing Interests: Declaration of interests JMZ has received consulting fees from Gilead Sciences and Merck & Co. JA has received consulting fees from Gilead Sciences; honoraria from Gilead Sciences and ViiV Healthcare; travel support from Gilead Sciences and ViiV Healthcare; book royalties from Elsevier Royalties; and has served on a Data Safety and Monitoring Board for the RIO HIV Trial. DDD has received consulting fees from Gilead Sciences and honoraria from Gilead Sciences and ViiV Healthcare. VDL has received research grants from the Canadian Foundation for AIDS Research and Canadian Institutes of Health Research and an honorarium from ViiV Healthcare. AC has received a research grant from Gilead Sciences. ND has received an honorarium from ViiV Healthcare. CD has received consulting fees from WHO. SH has received consulting fees from the BCG Boston Consulting Group. JVL has received consulting fees from Echosens, GlaxoSmithKline, Novo Nordisk, NovoVax, and Pfizer; honoraria from AbbVie, Echosens, Gilead Sciences, Janssen Therapeutics, Moderna, Merck Sharp & Dohme, Novo Nordisk, Pfizer; and has served on a Data Safety and Monitoring Board for the QuickStart Study. KM has received research grants and consulting fees from Gilead Sciences, Merck & Co, and ViiV Healthcare. MM has received consulting fees from Janssen Therapeutics. SRa has received an honorarium from the AIDS Healthcare Foundation. All other authors declare no competing interests.

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., Competing Interests: Competing interests: None declared.

In the first paper in this Series we assessed theoretical and empirical evidence and concluded that the health of people living in slums is a function not only of poverty but of intimately shared physical and social environments. In this paper we extend the theory of so-called neighbourhood effects. Slums offer high returns on investment because beneficial effects are shared across many people in densely populated neighbourhoods. Neighbourhood effects also help explain how and why the benefits of interventions vary between slum and non-slum spaces and between slums. We build on this spatial concept of slums to argue that, in all low-income and-middle-income countries, census tracts should henceforth be designated slum or non-slum both to inform local policy and as the basis for research surveys that build on censuses. We argue that slum health should be promoted as a topic of enquiry alongside poverty and health., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Massive slums have become major features of cities in many low-income and middle-income countries. Here, in the first in a Series of two papers, we discuss why slums are unhealthy places with especially high risks of infection and injury. We show that children are especially vulnerable, and that the combination of malnutrition and recurrent diarrhoea leads to stunted growth and longer-term effects on cognitive development. We find that the scientific literature on slum health is underdeveloped in comparison to urban health, and poverty and health. This shortcoming is important because health is affected by factors arising from the shared physical and social environment, which have effects beyond those of poverty alone. In the second paper we will consider what can be done to improve health and make recommendations for the development of slum health as a field of study., (Copyright © 2017 Elsevier Ltd. All rights reserved.)