This document provides a classification of defects that can occur in pharmaceutical products. It defines a defect as an imperfection that causes a product to fail to meet specifications. Defects are classified based on their measurability (variable or attributive), seriousness (critical, major, or minor), and nature (ocular, internal, or performance). The document also discusses defect classifications used by the Pharmaceutical Quality System and the UK Medicines and Healthcare products Regulatory Agency, and lists some common root causes of deficiencies in a pharmaceutical quality system.