Asthma and chronic obstructive pulmonary disease (COPD) are among the most prevalent lung diseases. In both asthma and COPD, airway inflammation leads to airway remodeling. Parenchyma of the lung is also influenced by disease conditions. Airway wall thickening/lumen narrowing and parenchymal destruction occur in COPD. In asthma, airway remodeling contributes to the lung parenchyma. Computed tomography (CT) has been widely used as an imaging tool for lung diseases. With the technical advancement of CT, together with the development of analysis software, it is now possible to analyze the lung parenchymal change and airway remodeling quantitatively using CT. This article reviews the role of CT in assessing the lung structure and functions of patients with asthma and COPD.