Depending on its origin or the particular batch, a given drug, although chemically well defined, may exhibit quite different behaviour. Process conditions (grinding, tabletting, granulation, drying) may also affect secondary (derived) properties of the drug, such as compactibility, wettability, solubility, dissolution rate, bioavailability and even pharmacological activity. Property changes may also be due to the storage conditions of the dosage form. In solids, these differences, which may exist initially or be generated by some treatment, arise directly from the primary characteristics of the crystals, i.e. the crystalline structure, both internal (polymorphism, pseudopolymorphism, amorphous state) and external (habitus, facies), crystal defects (dislocations) or the presence of impurities. In this respect, crystal « doping » by foreign substances may be envisaged as a means to improve drug properties of technological or biopharmaceutical interest. This review article explains these concepts using classical or more recently published examples from the literature.