Provision of integrated social services is not new. It is however, increasingly being seen as key to addressing service fragmentation and inefficiencies. But what do we know about how well social service integration works to improve outcomes for vulnerable groups of people, in New Zealand and internationally? This What Works brings together information on the effectiveness of integrated social services, including what is known about how, when, and for whom integrated social services are most effective. We also identify several factors to consider when deciding whether to implement integrated social services.