Traveller Communities constitute a complex population. A lack of reliable demographic data combined with nomadic lifestyles leads to unmet needs. Outreach has been utilized as a strategy to engage Traveller Communities in health improvement interventions. Using realist synthesis, a theory-driven approach which explores what it is about an intervention that works, for whom, in what circumstances, and why, we synthesized the available evidence on outreach interventions for Traveller Communities. We undertook comprehensive searches of electronic databases and gray literature to identify publications relevant to Traveller Communities and health. We then conducted expert hearings involving Traveller Community members and outreach workers to refine and validate our emerging findings. We found that outreach works best when community members feel they can trust the worker. Depending on this, and whether they have a say in the focus of the intervention (i.e., an intervention might aim to improve immunization rates, but if this isn’t seen as a priority it is less likely to be effective), outreach interventions can lead to three types of engagement: participation, behavior change, and social capital development. Our synthesis detailed the conditions making each most likely to happen. Realist synthesis enabled us to maximize the potential of this heterogeneous literature base, and expert hearings were key in producing a unifying explanatory framework based on trust and engagement.