Background: Improving the health of Traveller Communities is an international public health concern but there is little evidence on effective interventions. This study aimed to explain how, for whom and in what circumstances outreach works in Traveller Communities. Methods: A realist synthesis was conducted. Systematic literature searches were conducted between August and November 2011. Grey literature was sought and key stakeholders were involved throughout the review process. Iterative steps of data extraction, analysis and synthesis, followed by additional searches were undertaken. Results: An explanatory framework details how, why and in what circumstances participation, behaviour change or social capital development happened. The trust status of outreach workers is an important context of outreach interventions, in conjunction with their ability to negotiate the intervention focus. The higher the outreach worker’s trust status, the lower the imperative that they negotiate the intervention focus. A ‘menu’ of reasoning mechanisms is presented, leading to key engagement outcomes. Conclusions: Adopting a realist analysis, this study offers a framework with explanatory purchase as to the potential of outreach to improve health in marginalised groups.