Drug delivery by inhalation is the principal strategy to treat obstructive lung diseases, which affect about 15% of the population in the United States. Aerosol delivery devices have evolved over more than 60 years from the basic pressurized metered-dose inhaler and nebulizer to numerous types of inhalers and devices, including valved holder chambers, dry powder inhalers, soft mist inhalers, as well as smart inhalers and nebulizers. Although these devices improve a patient's ability to self-administer medication, many problems with optimal delivery still exist. Appropriate selection and repeated patient education can help lessen the problems with these devices. Aerosol science is evolving, with methods of measurement that include radio-scintigraphy and magnetic resonance imaging, to provide a better understanding of aerosol delivery and effects. Understanding the science and clinical application of aerosol drug delivery can substantially aid clinicians in optimizing these therapies for their patients.