This article reflects on a series of collaborative studies led by the author where co-investigation with peer-researchers has played a central role. The first concerns work with young people, trained to enable them to participate as peer-researchers in a child mobility study in Ghana, Malawi and South Africa; the second a research project on youth and mobile phones, in which some of those young peer-researchers have a continued involvement; the third a study of older people’s mobility in Tanzania, conducted in collaboration with an international NGO. Experience in these projects illustrates the complexities of co-investigation, not least the ethical concerns which have to be addressed when working with commonly marginalized people, whatever their age, but it also highlights the potential rewards which such collaborations can bring to individual peer-researchers, to academic research quality and, in the longer term, towards better policy and practice.