Infant and under-five mortality rates in low- and middle-income countries (LMIC) can be reduced by encouraging behaviours such as sleeping under insecticide-treated bed nets, exclusive breast-feeding for the first 6 months, regular handwashing, etc. Community-based volunteer or peer-to-peer mechanisms are cost-effective ways of promoting these lifesaving practices. However, the sustainability and reach of community-based behaviour change promotion remains a challenge. Our inquiry focuses on the utilisation, by non-governmental organisations (NGO), of Care Groups, a peer-to-peer behaviour change intervention. We asked: What are the mechanisms and contexts by which Care Groups achieve social and behavioural change in nutrition, health and other sectors? Design: Realist synthesis reviewing forty-two texts that contained empirical evidence about Care Group interventions. Setting: LMIC. Participants: We held consultations with a research reference group, which included Care Group and nutrition experts, and Care Group – implementing NGO staff in Malawi. Results: Different types of motivation drive the establishment and the sustainability of peer group interventions. A certain amount of motivation was derived from the resources provided by the NGO establishing the Care Groups. Subsequently, both volunteers and neighbourhood group members were motivated by the group dynamics and mutual support, as well as support from the wider community. Finally, volunteers and group members alike became self-motivated by their experience of being involved in group activities. Conclusions: When designing and implementing community-based behaviour change interventions, awareness of the multi-directional nature of the motivating drivers that are experienced by peer- or community group members is important, to optimise these groups’ reach and sustainability.