PET and PET-CT are rapidly evolving as modalities of thoracic imaging. In the mediastinum, PET can provide information to distinguish thymic hyperplasia from neoplasia, although the use of this imaging for this purpose is not accepted uniformly as necessary. PET is the standard of care in staging and follow-up of mediastinal lymphoma and in follow-up of metastatic seminomas after chemotherapy. Mycobacterial/fungal infections, sarcoidosis, and brown fat can mimic malignant findings on PET in the mediastinum.