This article synthesizes the evidence on the effectiveness of top-down and bottom-up approaches in providing basic services in slums in developing countries. The findings indicate that: (1) government-led top-down approaches focus predominantly on connectivity, but approaches led by community-based organizations (CBOs) and non-government organizations (NGOs) improve many additional dimensions of access; (2) legal recognition of slums improves access to services in both approaches; and (3) NGO and CBO involvement results in the right balance between the technical, social and financial resources required for effective service delivery. © The Authors 2017. Development Policy Review © 2017 Overseas Development Institute