In the pharmaceutical, chemical, and biotech industries, understanding the distinction between purity and assay is critical for quality control and regulatory compliance. Purity measures how free a substance is from impurities, while assay quantifies the actual amount of the target active compound. Although a product may be 99% pure, it could contain only 90% of the intended active ingredient due to degradation or the presence of inactive isomers. This article explains their definitions, calculation methods, analytical techniques, and real-world applications. It also explores how both metrics complement each other in formulation, manufacturing, and approval processes, helping industry professionals make informed decisions in material selection and product evaluation.