Since 1975 computed tomography (CT) has become available for the study of the body. In the thorax CT proves to be valuable to evaluate the lesions of the mediastinum, pleura and chest wall. However, the great difference in attenuation between air-filled lung and other soft tissue structures contributes greatly to the success of conventional diagnostic modalities, i.e. plain chest radiography and conventional tomography in the diagnosis and evaluation of the mass lesions in the lung parenchyma. And the role of CT is less valuable than in the other areas. However, CT is useful to diagnose and evaluate the chest wall or mediastinal invasion as well as mediastinal lymph node involvement. In radiotherapeutic managements of the malignancies in the thorax CT is valuable for the definition of the target volume and physical dose planning because CT provides information on patient outline and other internal structures of interest in addition to the size and shape of the tumor. CT is useful to estimate the efficacy of the conservative treatment, such as chemotherapy or radiotherapy for the thoracic malignancies, especially those in the mediastinum and chest wall. During follow-up study of the pulmonary malignancy conventional radiographic techniques may not be as valuable as before treatment because there is great change of the pulmonary architecture due to fibrosis and/or atelectasis secondary to the treatment. And CT may give valuable information in such cases.