Mediastinoscopy was initiated by E. Carlens in 1959 and enabled the exploration of the mediastinal tissues, above all the lymphatics with both a diagnostic and a prognostic aim. Surgical intervention was performed most often under general anaesthesia and cervical mediastinoscopy was carried out under direct visual inspection thanks to a thoracoscope with its own light source so that the mediastinal axis, the thymic area and also the peripheral mediastinum could be examined. The examination can be completed or even replaced by antero-lateral mediastinoscopy. Biopsies with forceps are in general of good quality. Side effects or complications are rare (1-3.8%) and the mortality is practically nil (0-0.5%). Diagnostic mediastinoscopy is indicated in cases of isolated or multiple mediastinal adenopathy, mediastinal tumours in particular those of the thymus whose extent is not evident. And finally certain miscellaneous pulmonary conditions. The sensitivity is 95-99% in sarcoidosis or metastatic lymphadenopathy and a little less in malignant lymphomas or thymic tumours. Mediastinoscopy is used to assess prognosis as regards the extension of broncho-pulmonary cancer. The existence and the site of metastatic nodes can be precisely assessed possibly on the contralateral side too, and influences the therapeutic orientation. The rare failures of mediastinoscopy leave a remaining 5% for an exploratory thoracotomy. Computered tomography does not seem to replace mediastinoscopy but rather guides its application. It is also part of the assessment of malignant lymphomas in certain situations.