Sharing data about patients between health and social care organisations and professionals, such as details of their medication, is essential to provide co-ordinated and person-centred care. While professionals can share data in a number of ways - for example, through shared electronic record systems or multidisciplinary team meetings - there are many factors that make sharing data across the health and social care boundary difficult. These include professional hierarchies, inaccessible electronic systems and concerns around confidentiality. Data-sharing is particularly important for the care of older people, as they are more likely to have multiple or long-term conditions; understanding is needed on how to enable effective data-sharing. To identify factors perceived as influencing effective data-sharing, including the successful adoption of interventions to improve data-sharing, between healthcare and social care organisations and professionals regarding the care of older people. MEDLINE and seven further databases were searched (in March 2023) for qualitative and mixed-methods studies. Relevant websites were searched and citation-chasing completed on included studies. Studies were included if they focused on older people, as defined by the study, and data-sharing, defined as the transfer of information between healthcare and social care organisations, or care professionals, regarding a patient, and were conducted in the United Kingdom. Purposive sampling was used to obtain a final set of studies which were analysed using framework synthesis. Quality appraisal was conducted using the Wallace checklist. Stakeholder and public and patient involvement groups were consulted throughout the project. Twenty-four studies were included; most scored highly on the quality appraisal checklist. Four main themes were identified. Within Goals, we found five purposes of data-sharing: joint (health and social care) assessment, integrated case management, transitions from hospital to home, for residents of care homes, and for palliative care. In Relationships, building interprofessional relationships, and therefore trust and respect, between professionals supported data-sharing, while the presence of professional prejudices and mistrust hindered it. Interorganisational Processes and procedures, such as a shared vision of care and operationalisation of formal agreements, for example data governance, supported data-sharing. Within Technology and infrastructure, the use of technology as a tool supported data-sharing, as did professionals' awareness of the wider care system. There were also specific factors influencing data-sharing related to its purpose; for example, there was a lack of legal frameworks in the area of palliative care. Data-sharing was usually discussed in the context of wider initiatives, for example integrated care, which meant the information provided was often limited. The COVID-19 pandemic has had significant impacts on ways of working; none of our included studies were conducted during or since the pandemic. Our findings indicate the importance of building interprofessional relationships and ensuring that professionals are able to share data in multiple ways. Exploration of the impact of new technologies and ways of working adopted as a result of the COVID-19 pandemic on data-sharing is needed. Additionally, research should explore patient experience and the prevention of digital exclusion among health and social care professionals. The protocol was registered on PROSPERO CRD42023416621. This award was funded by the National Institute for Health and Care Research (NIHR) Health and Social Care Delivery Research programme (NIHR award ref: NIHR135660), as part of a series of evidence syntheses under award NIHR130538, and is published in full in Health and Social Care Delivery Research; Vol. 12, No. 12. See the NIHR Funding and Awards website for further award information.