Classification of mediastinal masses as to location, incidence and types are presented. The orderly choice of diagnostic procedures will allow the diagnosis of most mediastinal masses. Two most useful diagnostic procedures are the scalene node biopsy and angiocardiography. But by the careful use of diagnostic studies, useless, unprepared and hazardous operations can be avoided. There will be instances however were thoracotomy will be necessary for diagnosis. In the management of a patient with a mediastinal mass procrastination in establishing the nature of the abnormality is not in the best interest of the patient. In this study 782 cases were encountered of which 42% proved histologically malignant. Illustrative examples are presented. Thoracotomy is recommended when other methods are not helpful. It has a low case fatality rate, offers cure in benign lesions, and gives accurate information on the histologic nature and the gross involvement by the tumors. This information permits intelligent management and provides valuable information for prognosis.