Systematic improvement of health services requires the objective measurement of people, practices and organisations against valid and explicit standards in order to identify and implement appropriate change. Effective quality systems must embrace a wide range of definitions of quality, and a similar variety of approaches to defining, measuring and improving. Clinical performance may be examined from three professional viewpoints--clinical competence: assessment of individual practitioners against explicit criteria to recognise achievement and to promote continuing development. Traditional mechanisms of training, registration and accreditation enable clinicians to reach career grades but responsibility for subsequent support is often unclear between employers, professions and registering bodies. Clinical practice: assessment of actual clinical process and outcomes against research-based "best practice" to identify and reduce variation. Peer review, clinical audit and confidential enquiries are examples of this approach, which may involve single or multiple professional groups and their interface with management. Service accreditation: systems to assess health care organisations against published standards in order to encourage best management practice. These are usually run on a regional or national basis and, though sensitive to expectations of patients, managers, clinicians, paying agencies and government, they are usually managed by an impartial but authoritative organisation.