Bronchial mucus has tomodensitometric features and MR signal intensity similar to that of water. However, chronic entrapped mucus collections, due to water reabsorption and higher protein content, can have CT attenuation values higher than 20 and reaching even 130 HU. Higher protein concentration also causes a sensible reduction in T1 relaxation time. The demonstration of mucus within a mediastinal, bronchial or pulmonary lesion is an important diagnostic clue permitting remarkable shortening of the list of differential diagnoses. This article illustrates the CT and MR findings allowing correct characterization of the mucus-containing lesions of mediastinum, bronchi, and lung.