The pressurized metered dose inhaler (pMDI) is currently the most popular inhalation dosage form for delivering drugs to the lungs in the treatment of asthma and other pulmonary diseases. Factors influencing the production of an effective respirable dose from a pMDI are the formulation and the primary and secondary atomization processes. The concern regarding the atmospheric effects of chlorofluorocarbon (CFC) propellants and the advent of alternative, hydrofluorocarbon (HFC) propellants has brought the atomization process to prominence. A review of the pMDI atomization processes is presented.