The mediastinum is a complex anatomical region with a wide range of differential diagnoses for masses arising in this region, including congenital, neoplastic, and inflammatory lesions. The classification for these has been based on anatomic compartment, according to the International Thymic Malignancy Interest Group (ITMIG) classification [1], which is to be applied to cross-sectional imaging studies. A detailed discussion of specific imaging features of each entity has been included in this chapter. In addition, a compartment-wise approach, based on salient imaging features has been provided as summary for each section.