The middle mediastinum contains structures like the heart, great vessels, trachea and lymph nodes. Common masses include lymphadenopathy, bronchogenic cysts, duplication cysts and aneurysms. Aortic aneurysms appear as round masses that may erode bone or involve nerves. Bronchogenic cysts are usually round and contain fluid. Duplication cysts may contain gastric mucosa. The document describes anatomy of the middle mediastinum and structures within it, common masses found and how they appear on imaging. It also discusses some pitfalls in interpretation like variants involving great vessels.