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Inhalation is one possible method of administering drugs into the human body. Drugs delivered in this manner reach different parts of the lungs in order to treat lung diseases. If a drug reaches the alveolar surface, it can also pass into systemic circulation – from the alveolar space through the respiratory epithelium, interstitium and capillary endothelium into the blood. Knowledge of factors which affect the deposition of aerosols in the airways is a crucial prerequisite for successful treatment. Such factors include aerosol particle or droplet properties, geometry and condition of the airways and type of breathing. Even if these factors are taken into account, less than half of a metered dose is transferred into the lungs when modern devices for aerosol formation are used (nebulizers, pressurized metered dose inhalers and dry powder inhalers). Designing of effective inhalers therefore remains a technological challenge. Goals include more effective treatment of lung diseases and systemic drug delivery as a non-invasive alternative, compared to injections, which need to be used for peptides or other labile molecules.