Purpose – One Stop Shops (OSSs) are a model of integrated delivery, where a range of needs are met in one place through a bespoke, person-centred service that works across organisational and sectoral boundaries. Third Sector Organisations provide OSSs for women in the criminal justice system and whilst they are shown to be effective in meeting complex needs, how this effectiveness is generated needs explaining. The purpose of this paper is to set out a preliminary framework for explaining their effectiveness. Design/methodology/approach – A realist review was conducted to research the way in which context influences how OSSs work. This approach was chosen because it is designed to synthesise evidence on social programmes and policies which are complex. The review was conducted in two phases: theory building and theory refinement. Findings – The review explains how individual, interpersonal, institutional and infrastructural contexts influence the effectiveness of OSSs. The evidence suggested that the organisational or legal form of the provider is not as important as the values and attitudes of their staff. Originality/value – The paper adds to our understanding of how OSS services work, and gives insight for commissioners and Third Sector providers of services by beginning to explain how this effectiveness is generated.