Providing water and sanitation services to low-income communities is a key development challenge in urban areas. In Buenos Aires, the delivery of services to low-income neighbourhoods has historically been undermined by changing combinations of political clientelism, poorly coordinated government programmes, inappropriate and eventually rescinded privatization schemes and a lack of participation by local people. In this changing context, the Instituto Internacional de Medio Ambiente y Desarrollo (IIED–América Latina) has been working with communities, local authorities and utilities to secure access to water and sanitation for unserved communities. This paper reflects on the experience and how it demonstrates both the importance of local action within communities when public service delivery is inadequate, and how important an understanding of the situation in local communities can be when service providers are attempting to extend their delivery to informal and unserved settlements. The paper traces the different stages of IIED–AL’s work, explaining the challenges encountered and the strategies used to overcome them. It also examines some key determinants of success, namely working in collaboration with other actors, including local government; using information as a driver for change; finance mechanisms; and strategies for scaling up.