Clinical audit is a sustained cyclical quality improvement process seeking to improve patient care and outcomes by evaluating services against explicit standards and implementing necessary changes. National audits aim to improve population-level clinical care by identifying unwarranted variations and making recommendations for clinicians, managers and service commissioners. The National Clinical Audit of Anxiety and Depression aimed to improve clinical care for people admitted to English hospitals for treatment of anxiety and depression, to provide comparative data on quality of care, and to support local quality improvement initiatives by identifying and sharing examples of best practice.