This chapter discusses the nature of error, the classification of error, concepts in quality management, types of error mitigation, hierarchy of interventions, and the role of culture in error prevention. Psychologists have proposed a dual system processing theory for thought. More recently, a classification system based on Reason’s error categorization, Human Factors Analysis and Classification System (HFACS), has been demonstrated to improve standardization of error classification, increase the specificity of error classification, and facilitate the development of actionable corrective plans. Readers are directed to the guidance from the American Society for Radiation Oncology (ASTRO) on this subject for a detailed treatise on the components of peer review. An effective quality management program depends on a robust understanding of error, appropriate classification of error, the application of a systems approach to problem solving, the careful choice and implementation of error mitigation strategies, and a commitment to continuous improvement.