As the number of older people in the UK population increases, with resulting increases in acute admissions to hospital, there is concern that the NHS and health systems in other countries will not be able to respond with timely and high-quality healthcare. In response there is ongoing and completed research, including an NIHR-funded multi-centre randomised trial, to see whether geriatrician-led care in the patient’s home is an effective alternative to hospital admission. This is referred to as Admission Avoidance Hospital at Home (HAH). This PhD has at its centre, a trial of 1055 older people who were referred to hospital for an acute medical event and were randomised to either HAH or hospital care. This project used modern quantitative methods, randomised trial data and observational data to explore this intervention in depth, in relation to the main outcomes (living at home, residential care) and key secondary outcomes (delirium, readmission). A systematic literature review identified 4 new studies and provided evidence on the effectiveness of HAH, in particular in reducing the risk of long-term residential care. Multi-state modelling and causal mediation analysis of the trial data identified delirium as an effect modifier and potential mediator of the effect of HAH on risk of residential care. Using observational data we found overlap in characteristics between HAH patients and a target population of hospitalised patients, and found that HAH may make no difference to mortality compared with hospital care in the eligible target population. Finally, we showed using novel conceptual pathway methods how to illustrate a HAH patient journey, and identified barriers to scaling up HAH in the UK more widely, including sustainable financing and recruitment of staff.