Dr. Shephard reviews the current status of aerosol therapy with particular reference to the treatment of cystic fibrosis. The water deficit of the atmosphere, the composition and size of the aerosol particles and the mode of administration are shown to be important variables. The majority of aerosol generators create polydisperse clouds with a high mass median diameter (4-5 microns) and as much as 90 percent of the aerosol is deposited in the mouth and throat; in consequence, the volume of nebulized fluid reaching the bronchi is not more than 2-3 ml./hour.The importance of the dual blood supply to the lung is emphasized; if pharmacologically active aerosols are absorbed into the bronchial circulation, they may circulate as a "slug" and produce dangerous toxic effects. The influence of airflow turbulence upon the impaction of large particles, and a dead space upon the retention of small particles is considered. The main potential value of aerosol therapy lies in the application of high concentrations of active drugs to poorly vascularized areas of the lung. In order to realize this potential, better aerosol generators must be developed and much more must be discovered about the behavior of small particles in the airstream.