Chylothorax is a rare cause of pleural effusion, secondary to accumulation of lymph in the pleural space. Diagnosis is based on the triglyceride and cholesterol content of pleural fluid obtained with thoracentesis. Because the lymphatic system plays an essential role in fat absorption and immune response, lymphatic leak associated with chylothorax may cause life-threatening malnutrition and immunodeficiency. Chylothorax is usually described as traumatic or nontraumatic. The main cause of chylothorax is traumatic, typically postsurgical, secondary to iatrogenic direct puncture of the thoracic duct during thoracic surgery. Causes of nontraumatic chylothorax include a wide range of differential diagnoses. Lymphoma and thoracic malignancies are the most common causes and are responsible for chylothorax by extrinsic compression or invasion of the thoracic duct. Other rare causes include primary and secondary diffuse lymphatic diseases, responsible for chylothorax by lymphatic vessel wall dysfunction. Imaging the lymphatic system remains a challenge in the days of modern imaging. Nonenhanced MR lymphography is a noninvasive technique based on heavily T2-weighted sequences, thus enabling visualization of the lymphatic circulation. This technique allows diagnosis and differential diagnosis, evaluation of disease severity, and guidance of therapeutic management in nontraumatic chylothorax. Furthermore, it may offer radiologic classification of primary lymphatic diseases on the basis of morphologic features of lymphatic vessels. The authors describe the anatomy and physiology of the thoracic lymphatic system, present the technique of nonenhanced MR lymphography, and discuss pathophysiologic mechanisms and imaging features in different causes of nontraumatic chylothorax. ©RSNA, 2020.