The NHS needs locum doctors. They are a key component of the medical workforce in the NHS and provide necessary flexibility and additional capacity for healthcare organisations and services. But they should be used appropriately, and where they are used, they should be supported effectively. The number of doctors working as locums, and the costs of this to the NHS have caused some concerns nationally in recent years. It has also been suggested that locum doctors may not provide as good a quality of care as permanent doctors. So, we set out to find out more, through two large surveys of NHS trusts and general practices in England; interviews and focus group discussions with staff in NHS organisations and with locums, locum agencies and patients; and by analysing data that had already been collected about the NHS workforce and about clinical care. This report presents our findings and their implications.