More than half of humanity currently lives in urban areas and 90% are projected to do so by 2050. One in three urban dwellers, and an estimated 881 million people in Low and Middle-Income Countries (LMICs) now live in informal settlements, colloquially known as ‘slums’, where they have inadequate, insecure housing and limited services. These settlements are often the only option for people working in the informal sector, and existing evidence shows that residents are often highly resourceful, resilient and innovative in navigating the problems they face daily. However, they also face challenges that appear intractable: multiple health and well-being risks and vulnerabilities, related to an unhealthy environment, insecurity and social marginalisation. Informal urban settlements are rapidly changing, with in and outmigration and shifts in social norms, as well as being economically and socially unequal. Failures to provide services, infrastructure and mechanisms for redress by national and local governments are often linked to the presence of multiple actors and organisations including non-governmental, private and criminal organisations. These conditions can act as an incubator for various forms of violence (e.g. political and gender-based), which threaten well-being.