Complex particulate systems are the basis for all solid dosage forms, starting with the active pharmaceutical ingredient, which has to be produced in the solid state in the first instance, most often by crystallization from solution. The drug is then processed to a form that has utility in the final dosage form, initially contributing to flow and dispersion and finally in terms of release of drug. Characterization of the physicochemical properties of both the starting and processed material is essential to maintaining control of the final product. A variety of methods of defining the properties of the pharmaceutical particulate system and of manufacturing particles and the implications for their performance have been described.