The main principles underpinning measurement for healthcare improvement are outlined in this Element. Although there is no single formula for achieving optimal measurement to support improvement, a fundamental principle is the importance of using multiple measures and approaches to gathering data. Using a single measure falls short in capturing the multifaceted aspects of care across diverse patient populations, as well as all the intended and unintended consequences of improvement interventions within various quality domains. Even within a single domain, improvement efforts can succeed in several ways and go wrong in others. Therefore, a family of measures is usually necessary. Clearly communicating a plausible theory outlining how an intervention will lead to desired outcomes informs decisions about the scope and types of measurement used. Improvement teams must tread carefully to avoid imposing undue burdens on patients, clinicians, or organisations. This title is also available as Open Access on Cambridge Core.