This chapter explores the foundations of nonconformance investigation, classification, and control within a Quality by Design (QbD) framework. It distinguishes key process elements, parameters, and attributes and examines the decision-making aspects and classification methodology. The chapter provides step-by-step approaches to managing nonconformances, which include deviation and failure observations. It highlights the criticality of rapid action, nonconformance classification, notification, immediate data gathering, and data integrity requirements. Robust data collection, immediate containment actions, and data-driven impact assessment practices enable organizations to identify root causes proactively and ensure defining the appropriate corrective and preventive actions.