Segmental or lobar atelectasis is a common radiographic finding in patients with bronchial obstruction. Although in many cases the presence and cause of atelectasis can be ascertained from the plain chest radiograph, computed tomography (CT) often provides valuable additional information, particularly with regard to the precise location and extent of the obstructing process. CT also can be used to differentiate between benign and malignant causes of obstructive atelectasis. After administration of intravenous contrast material, CT may distinguish a proximal obstructing tumor from collapsed lung or adjacent mediastinal structures. In some patients, magnetic resonance imaging (MRI) can provide comparable and/or complementary information. In this article, we review the major applications of CT and MRI, as well as the capabilities and limitations of both techniques, in the evaluation of patients with obstructive atelectasis.