Mediastinal masses represent a vast group of tumours and pseudo-tumours which can involve the various compartments of the mediastinum. The authors propose a radiologic diagnostic approach starting from the plain thoracic radiograph with study of the mediastinal lines and oesophageal transit and going on to the classifications made possible by modern CT and MR imaging. The proposed diagnostic procedure is based on nine mediastinal lines and two 'threads of Ariadne' which are the compartments where the masses are located and their behaviour at CT (densitometry before and after administration of an iodinated bolus) and at MRI (T1, T2, gadolinium-enhanced T1-weighted sequences). The definitive aetiological diagnosis may be established by surgery, but also in certain cases by percutaneous needle biopsy.