There is a growing concern that adverse events occur frequently in operating theatres. Adverse events such as wrong site surgery and surgical site infections have a severe detrimental impact on not only the patient but also the healthcare staff and the services. Institute of Medicine’s report, ‘To err is human’ highlighted that teamwork failures are a leading cause of death and suffering. Yet, in surgery, measuring teamwork and designing interventions to improve teamwork and patient safety in operating theatres remains an area of research that is largely unexplored. This thesis aims to measure and improve teamwork in operating theatres to ensure safer surgery. In this project, the WHO surgical safety checklist was evaluated for its impact on patient safety. The WHO checklist improved patient safety processes in operating theatres but its impact on teamwork, intra-operative problems and theatre efficiency was not clearly understood. Therefore, a framework was developed to measure teamwork failures, equipment problems and technical failures as surrogate markers of teamwork, patient safety and efficiency in operating theatres. Equipment failures emerged as a sensitive measure of teamwork in operating theatres. Teamwork failures were also associated with technical failures, delay in case progress and patient harm. It emerged that the WHO checklist can improve teamwork and theatre efficiency and reduce equipment problems in operating theatres when it is used in its true spirit rather than a tick-box exercise.