In chemistry, impurities are chemical substances that are present inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound. Impurities can be either naturally occurring or added during the synthesis of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance. Impurities can be destructive when they obstruct the working nature of the material. For example, ash and debris in metals and leaf pieces in blank white papers.

There are different types of impurities in pharmaceuticals, including organic impurities, inorganic impurities, and residual solvents. Organic impurities are often process-related or drug-related pharmaceutical impurities, such as starting materials, by-products, intermediates, degradation products, reagents, ligands, and catalysts. Inorganic impurities often derive from the manufacturing process, such as reagents, ligands, catalysts, heavy or residual metals, inorganic salts, filter aids, or charcoals. Residual solvents are residuals of solvents present in the manufacturing process.

The level of impurities in a material is generally defined in relative terms, and standards have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. It is usually impossible to separate an impurity completely from a material, as removing impurities completely means reducing their concentration to zero, which would require an infinite amount of work and energy as predicted by the second law of thermodynamics. Impurities play an important role in the nucleation of other phase transitions, and the presence of foreign elements may have important effects on the mechanical and magnetic properties of materials.