Background: Hospital at Home (HaH) provides intensive, hospital-level care in private or care homes for acute conditions that would normally require an acute hospital bed, using multidisciplinary healthcare teams. As a programme of complex medical-social interventions, a HaH programme theory has not been fully articulated yet, although implicit in the structures, functions, and activities of the existing HaH services. We aimed to unearth the tacit theory from international evidence and test the soundness of the theory by studying UK HaH services.Methods: We conducted a rapid systematic review of 29 articles adopting a ‘realist review’ approach and examined 11 UK-based services by interviewing up to 3 staff members (e.g. doctors, nurses, service leads, therapists) from each service. The review and interview data were analysed using Framework Analysis and Purposive Text Analysis.Results: Our programme theory has three interlinked components: Organisation, Utilisation and Impact. Central to the programme theory is Impact Theory. It shows multifaceted impacts of HaH on patients and carers and at the system level; and reveals multiple, intertwined pathways of change in which certain outputs of the main HaH functions and activities were the instigating causes and certain clinical, health and wellbeing, and system-level benefits were the effects they eventually produced. Our findings also highlight the main features of HaH that created added value for patients and carers: comprehensiveness of assessment leading to individually tailored, situational appropriate interventions; special and more holistic healing effects of home environment; high quality interactions between professionals and patients and carers; and less complexity and fragmentation of care.Conclusions: We have shown that the multifaceted impacts of HaH add value to patient care, carer support and health system performance, making HaH a better option for some patients who would otherwise need hospital admission. Our Impact Theory helps unpack the “black box” to reveal how these impacts can be brought about, informing how HaH services can organise resources and design processes of care to optimise patient satisfaction and outcomes. It can be used as a basis for formulating and prioritising evaluation questions, designing evaluation research, and interpreting evaluation findings in future HaH service evaluations.