Background: Locum doctors give practices flexibility to deliver patient services but there are concerns about the impact of locum working on continuity of care, patient safety, team function and cost. Aim: To explore locum working in English general practices and understand why and where locum doctors were needed and how they were engaged, supported, perceived and managed. Design and Setting: An online survey was sent to 3745 practices. Methods: Quantitative responses analysed using frequency tables, t-tests and correlations. Free-text responses were analysed using thematic analysis. Results: 605 responses were returned (16.2%). Practices made frequent use of locums, preferring regular locums familiar with processes and patients. Disadvantages of agency locums included cost, lack of patient familiarity and impact on continuity of care. Care provided by locums was generally viewed as the same but sometimes worse compared to permanent GPs. Some practices reported that locums did not always perform the full range of duties resulting in increased workload for other staff. Practices were largely unfamiliar with national guidance for organisations engaging locums and while processes like verifying documentation were conducted, far fewer respondents reported providing feedback, support for revalidation, or professional development. Conclusion: Locum working is an essential part of English general practice, but our research raises some concerns about the robustness of arrangements for locum working and impact on quality and safety of care. Further research is needed on the clinical practice and performance of locums, and to explore how locum working can be organised in ways that assure safe and high quality care.