Emergency healthcare services are under intense pressure to meet increasing patient demands. Many patients presenting to emergency departments could be managed by general practitioners in general practitioner-emergency department service models. To evaluate the effectiveness, safety, patient experience and system implications of the different general practitioner-emergency department models. Mixed-methods realist evaluation. Phase 1 (2017-8), to understand current practice: rapid realist literature review, national survey and follow-up key informant interviews, national stakeholder event and safety data analysis. Phase 2 (2018-21), to collect and analyse qualitative (observations, interviews) and quantitative data (time series analysis); cost-consequences analysis of routine data; and case site data for 'marker condition' analysis from a purposive sample of 13 case sites in England and Wales. Phase 3 (2021-2), to conduct mixed-methods analysis for programme theory and toolkit development. General practitioners commonly work in emergency departments, but delivery models vary widely in terms of the scope of the general practitioner role and the scale of the general practitioner service. We developed a taxonomy to describe general practitioner-emergency department service models (Integrated with the emergency department service, Parallel within the emergency department, Outside the emergency department on the hospital site) and present a programme theory as principal output of the study to describe how these service models were observed to operate. Routine data were of variable quality, limiting our analysis. Time series analysis demonstrated trends across intervention sites for: increased time spent in the emergency department; increased emergency department attendances and reattendances; and mixed results for hospital admissions. Evidence on patient experience was limited but broadly supportive; we identified department-level processes to optimise the safety of general practitioner-emergency department models. The quality, heterogeneity and extent of routine emergency department data collection during the study period limited the conclusions. Recruitment was limited by criteria for case sites (time series requirements) and individual patients (with 'marker conditions'). Pandemic and other pressures limited data collection for marker condition analysis. Data collected and analysed were pre pandemic; new approaches such as 'telephone first' and their relevance to our findings remains unexplored. Findings suggest that general practitioner-emergency department service models do not meet the aim of reducing the overall emergency department waiting times and improving patient flow with limited evidence of cost savings. Qualitative data indicated that general practitioners were often valued as members of the wider emergency department team. We have developed a toolkit, based on our findings, to provide guidance for implementing and delivering general practitioner-emergency department services. The emergency care data set has since been introduced across England to help standardise data collection to facilitate further research. We would advocate the systematic capture of patient experience measures and patient-reported outcome measures as part of routine care. More could be done to support the development of the general practitioner in emergency department role, including a core set of competencies and governance structure, to reflect the different general practitioner-emergency department models and to evaluate the effectiveness and cost effectiveness to guide future policy. This study is registered as PROSPERO CRD42017069741. This award was funded by the National Institute for Health and Care Research (NIHR) Health and Social Care Delivery Research programme (NIHR award ref: 15/145/04) and is published in full in Health and Social Care Delivery Research; Vol. 12, No. 10. See the NIHR Funding and Awards website for further award information.