In most clinical situations the modern radiological approach to a mediastinal mass consists of performing a CT scan following the chest radiograph. Magnetic resonance imaging is indicated when CT findings are equivocal and as the first-line method in particular situations such as suspected involvement of the posterior mediastinum. In both techniques, tissular components of the mass assessed by density or signal intensity analysis, together with the precise location, are the leading edge of the radiological diagnosis. This review deals mainly with the differential diagnosis of primary neoplasms according to CT and MRI findings.