The relative concentration of various impurities must be measured during the manufacturing process for biologic drugs such as therapeutic proteins. An impurity in a drug product is any component other than the drug substance itself, or a component of its formulation. This interactive scrollable explains why therapeutic proteins must be tested for impurities, describes the types of impurities that are tested for in the manufacturing of a therapeutic protein, and provides an overview of the analytical techniques that can be used to screen for various impurities.