BackgroundPhysical activity (PA) is critical in the prevention and treatment of non-communicable disease (NCD). Despite benefits to the health and wellbeing of individuals, the economy and the environment, a quarter of adults remain physically inactive globally. Despite increasing research, National and International policies and myriad interventions, this trend continues to rise. Recent calls have been made for the implementation of systems approaches to address the inactivity pandemic. Following the 2012 Olympic Legacy, the National Centre for Sport and Exercise Medicine (NCSEM) in Sheffield re-located NHS clinics within leisure centres to embed PA into treatment pathways as part of a whole system approach. With this in mind, this realist review aimed to develop theory and bring to light empirical data to explain how, why and under what circumstances co-location of health services in leisure facilities might lead to increased PA promotion and behaviour.MethodsA realist review was conducted as the first phase of the development of initial theories to identify how co-location might facilitate PA promotion as part of a wider realist evaluation (RE). Key concepts including “health clinic,” “leisure centre” and “co-located” were searched using a series of related terms via academic databases: MEDLINE, CINAHL, SportDiscus, SCOPUS and PsychInfo. Other sources included grey literature and stakeholder suggestions. The search yielded 1788 results. Documents that met search criteria were refined through critical discussion with the research team. A total of 39 documents were synthesised into 9 theories.ResultsEmerging theories to explain how co-locating health and leisure might lead to increases in PA emerged at varying levels of a system. Theories at an institutional level suggest purpose-built facilities create a physical environment that re-enforces a PA culture and supports behaviour change. A “one-stop” shop provides the opportunity for frictionless transition between health and leisure for patients. At an interpersonal level, theories include improvement in coordination, collaboration and communication between professionals and patients, and enhanced opportunity for multidisciplinary working. Theories at an individual level identified increased convenience, enhanced awareness of PA and improved experience. Theories which might explain the challenges of embedding PA in the NHS pertain to the logistics of service delivery and clinical instability. ConclusionsTo address physical inactivity, innovative approaches across the health and care system are required. The theories identified in this review provide a framework for healthcare, fitness professionals and planners to consider co-location of services as one such innovation for PA promotion. Theories identified in this review will be tested and refined through realist evaluation, including interviews with service users and healthcare professionals.