Impurities in pharmaceuticals are the unwanted chemicals that remain with the active pharmaceutical ingredient (API), or develop during formulation, or upon aging of both API and formulated products. Th e presence of these unwanted chemicals even in small amounts may infl uence the effi cacy and safety of the pharmaceutical products. According to the International Conference on Harmonization (ICH) of Technical Requirements for Registration of Pharmaceuticals for Human Use, impurities are classifi ed as organic impurities, inorganic impurities and residual solvents. Impurities can originate from several sources, such as starting materials, synthetic intermediates and by-products, degradation products arising during storage, functional group-related typical degradation, mutual interaction amongst ingredients and formulation- related impurities. Control of stereochemical purity of an API with chiral centers is also necessary. Also, polymorphic forms are sometimes considered as impurities that need to be controlled. Impurity profi ling is the common name of a group of analytical activities, the aim of which is the detection, identifi cation/structure elucidation and quantitative determination of organic and inorganic impurities, as well as residual solvents in bulk drugs and pharmaceutical formulations. Since this is the best way to characterize the quality and stability of bulk drugs and pharmaceutical formulations, this is the core activity in modern drug analysis. Highly sophisticated analytical techniques, such as chromatographic, spectroscopic and hyphenated techniques, are inevitable tools in the detection, identification, characterization and quantification of impurities.