Aims: To reduce costly acute hospital admissions of a growing elderly population, a number of complex interventions have been developed, but evidence of the effectiveness of these is inconclusive. What is it about these models that make them effective or ineffective? The aim of this study was to examine the research underpinning these models; analyse treatment components and identify what is and is not effective. Method: Thirteen international studies of complex interventions that reported hospital admission data as an outcome, published between 2000 and 2009, were examined. Findings: What works in reducing admissions is not one, but a combination of components, underpinned by the delivery of interventions by established, integrated health and social care teams. Conclusions: the most effective models in preventing older people being admitted to hospital are provided by established, integrated teams in the patient's home. However, cost effectiveness must be considered if effective interventions are to be delivered to a growing population of older people.