Delivery of drugs by aerosol is increasingly used as a means of providing therapy to both the lungs and to the rest of the body through absorption into the pulmonary circulation. The techniques, definitions, and terminology for aerosol delivery are not standardized or necessarily understood by the increasing number of clinicians and clinical investigators involved in these studies. This paper attempts to establish some definitions and guidelines for defining and characterizing deposition of aerosols in the respiratory tract. Some of the parameters that affect deposition are discussed (e.g., particle size, breathing pattern, airway caliber). It is suggested that these parameters be controlled or measured when characterizing new aerosol delivery systems for clinical investigations. These recommendations were discussed and agreed upon during an expert meeting on definitions and standards related to aerosols in medicine at the 8th Congress of ISAM.