Idiopathic chylothorax is a debilitating disease that can lead to respiratory and metabolic compromise and fibrosing pleuritis. Previous investigation has provided theories for the etiology of this poorly understood disease. This article provides an overview of the pathophysiology and diagnosis of chylothorax. Thoracic duct imaging, including minimally invasive techniques, is also discussed, as it is frequently performed in the perioperative period. A companion article reviews nonsurgical and surgical techniques for treating and managing idiopathic chylothorax in dogs and cats.