Aims: This proposal aims to conduct a realist evidence synthesis with embedded co-production and co-design, to provide evidence-based recommendations about how best to help people with long-term conditions, in the primary care setting, to maintain and improve their physical function and become more physically active. Background: Long-term conditions comprise the biggest burden on the NHS and primary care services. 'Physical function' can be defined as an individual's capacity to undertake the physical tasks of everyday living. People with long-term conditions often have reduced physical functioning and become less able to live independently and, despite the clear benefits for health and functioning, are also less physically active. Long-term conditions are managed routinely in primary care, with assessment and treatment aligning closely to the medical model of illness. However, physical function and physical activity are often not given as much priority. Psychosocial factors influencing physical function and physical activity also need consideration. A new way is needed to address the decline in physical function and physical activity in people with long-term conditions as part of primary care service delivery. Methods: This realist synthesis will use established methods and will embed the principles of co-production and co-design. Stage 1 will develop initial programme theory based on a scoping search of the scientific and grey literature, as well as two stakeholder workshops to generate initial ideas and theories. A stakeholder analysis will identify relevant stakeholders. These are likely to include patients, GPs, practice nurses, therapists and others involved in improving physical function and promoting physical activity. Stage 2 involves more focussed literature searching, data extraction and synthesis to provide evidence to support the initial programme theories. We will focus our searching on physical activity interventions that are part of primary care service delivery and include assessment of physical function. However, we will iteratively broaden our searches to include relevant areas such as studies from secondary care, the voluntary sector and social care. We will tease out the complexities relating to whether or not something works, for whom and in what circumstances and develop (C)ontext, (M)echanism and (O)utcome (CMO) configurations. A taxonomy of physical activity interventions suitable for optimising physical function for people with long-term conditions will be produced. Stage 3 involves testing and refining the programme theories. We will seek additional input from stakeholders in order to refine the final programme theories. The resulting theories will feed into Stage 4. This stage involves three sequential co-design stakeholder workshops where ideas for service innovation will be developed. A final knowledge mobilisation event will take place in order to address issues relevant to wider implementation to ensure that the intervention is relevant, useable and accessible to stakeholders. It will provide the content for a suite of dissemination materials (such as short video), for development by the design team. Dissemination: A protocol and overall findings paper will be written and published in a scientific journal. The materials from the knowledge mobilisation event will be used for targeted dissemination to relevant groups. PPI representatives will be invited to facilitate dissemination activities.