It is difficult to distinguish the various forms of tumor of the thorax and chest wall clinically. It is first important to differentiate benign and malignant tumors. In many instances, the clinical history and physical manifestations of the tumor suggest the type of growth, but histologic examination of the tissue must usually precede accurate diagnosis. The malignant tumors may be considered in two groups: those that primarily involve the structure of the thorax and chest wall, and those that secondarily involve these structures either by continuity or by metastasis from a primary growth elsewhere in the body. In the latter group, carcinoma of the breast is probably the most common primary lesion. Carcinoma of the thyroid not uncommonly invades the anterior mediastinum and manubrium. However, the primary lesion may arise in any part of the body. In this article, I have not included the metastatic malignant tumors or intrathoracic goiter.