Achieving and delivering optimal quality of care in radiology requires continual self-examination by the profession, particularly with regard to technical, interpretive, and communication skills. The importance of empirical data pertaining to quality and variability in radiology, the underlying causes of error, and the sources of variability are discussed. Key measures (e.g., receiver operating characteristics, kappa) and approaches (professional audits and peer reviews, surveys, inspections, and risk management programs) used in improvement efforts are reviewed, and data from key studies are highlighted. Diagnostic errors are important because of their connection to outcomes and the wide variability observed with modalities such as chest radiography and mammography.