When a mediastinal disease is suspected, the conventional chest X-ray remains the diagnostic procedure of first choice. However, the gold standard for evaluation of the mediastinum is represented by thoracic computed tomography, which demonstrates all important structures of this region and supplies information about pathologic changes of the lung hilus and parenchyma. Until today, with the exception of echocardiography, noninvasive sonographic examination of the mediastinum is not routinely performed. The potential of this procedure, which is supported by the good accessibility to the region and its predominantly solid structure, appears not yet adequately used. To evaluate and compare the sonographic findings, it seems essential to establish a standardized examination procedure as well as a clear definition of anatomic regions. Since lymphadenopathy represents the most frequent pathologic finding in the mediastinum, the definition of the mediastinal regions is made with respect to the lymphatic pathways. Clinically relevant mediastinal lesions are predominantly located in the aortopulmonary window or the paratracheal region, which both permit facile access for sonographic examination. A transcutaneous diagnostic or therapeutic puncture of mediastinal lesions can safely be performed under sonographic guidance.