The imaging techniques used to investigate patients with asthma and to assess the effects of asthma treatments include computed tomography (CT), helium magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron-emission tomography (PET). Only MRI does not involve radiation exposure. Technical improvements in CT, together with the imaging advantages inherent in the presence of air in the lung, have diminished the radiation exposure required for lung CT. High-resolution low-dose lung CT protocols deliver a dose roughly equal to 1 year of natural radiation exposure and can be used even in paediatric patients. To date, CT is the most extensively studied lung imaging method, the simplest to perform, and the least expensive. In patients with asthma, CT may show several structural changes related to small-airway disease including cylindrical bronchiolectasis, bronchial wall thickening, and air trapping; an indirect marker for bronchiolar obstruction. A robust body of evidence indicates that valid CT markers for small-airway disease can be derived from quantitative lung density measurements and that these markers correlate with clinical severity and lung function test results. In addition, these CT markers are sufficiently sensitive to demonstrate therapeutic effects.