We discuss the efficacy and limitations of CT and MRI in the assessment of the pleura and pleural cavity. With CT and MRI findings from 610 patients, we address six topics: 1) Normal anatomy of the pleural cavity, 2) Pleural effusion, 3) Pleural tumors, 4) Pyothorax, 5) Lung cancer, and 6) The pleura as a mirror of systemic diseases. CT showed specific findings of acute bleeding. MRI was sensitive enough for the diagnosis of subacute bleeding and chylothorax. In the evaluation of pleural tumors, CT and MRI revealed specific evidence of pedunculated localized fibrous tumors and lipomas. In long-standing pyothorax, it is important to detect any associated secondary malignancies. CT and MRI show as a mass penetrating the bony thorax. B-cell non-Hodgkin malignant lymphoma is the most commonly associated malignancy. The sensitivities for diagnosis of pleural extension of bronchogenic carcinoma are 80% with CT and 86% with MRI. The specificities are 89% with CT and 75% with MRI. To study the pleura as a mirror of systemic diseases, we used high resolution CT to examine 104 patients without evidence of the diseases mentioned above. Eighteen of those 104 patients were found to have pleural thickening, and all 18 were smokers. Of the 24 non-smokers, none had pleural thickening. The difference in the occurrence of pleural thickening between smokers and non-smokers was significant (p < 0.01).