Massive slums have become major features of cities in many low-income and middle-income countries. Here, in the fi rst 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 eff ects on cognitive development. We fi nd that the scientifi c literature on slum health is underdeveloped in comparison to urban health, and poverty and health.