The term small airways (SA) applies to the bronchi below 7th generation with diameter smaller than 2 mm. This paper presents data showing that this part of the respiratory system is distinct in terms of its architecture, physiology and pathophysiology. The most important role SA play in obstructive airway diseases. In healthy subjects SA resistance accounts for 10% of the total airway resistance while in patients with obstructive disease, due to the constriction of the airways together with airway inflammation, SA are responsible even for 60% of the total resistance. Changes in SA in asthma and COPD are responsible for air trapping especially prominent in the latter disease. There are no precise tools to diagnose SA. Depending availability and experience HRCT, body pletysmography (RV/TLS plus other parameters) are frequently used. Some hope for the future is placed in combined use of oscilometry, multiple (or single) breath nitrogen wash-out and eNO concentration measurement. Due to our increasing knowledge on the role of SA in airway obstruction ultra-fine particle aerosols have been developed that penetrate to this compartment of the respiratory system (deposition confirmed in cascade impactors as well as by ozone scans after aerosol inhalation). Authors present selected publications investigating whereas deep drug penetration influences its clinical efficacy. For LABAs deposition in SA doesn't seem to increase their bronchodilating effect. Using ultrafine CS aerosol allows to maintain clinical effects even with half the dose of the active steroid, however. In summary, SA seem to be crucial in obstructive diseases of the airways and therefore constitute an important target for therapy.