Although high-resolution CT scan has proved most useful in the diagnosis of infiltrative lung disease, its use in the diagnosis of airway and obstructive lung diseases has recently been emphasized. In particular, the use of dynamic expiratory or postexpiratory CT scans, usually in combination with an inspiratory high-resolution CT scan study, has proved useful in the diagnosis and assessment of obstructive lung diseases. This article reviews the use of expiratory CT scan in the diagnosis of lung disease, including the various CT scan techniques that can be used, normal and abnormal expiratory CT scan findings, and the use of expiratory CT scan in a variety of obstructive diseases.