Chylothorax is the term used to describe the accumulation of chyle in the pleural cavity. The condition results from a disruption of the thoracic duct, the principal lymphatic vessel within the chest, or its feeding lymphatics. A cholesterol pleural effusion (also called pseudochlothorax or a chyliform effusion) results when cholesterol crystals crystallize in pleural fluid that is supersaturated with lipids. The exact pathophysiology is unclear. A milky pleural effusion is generally the finding that first arouses the suspicion for chylothorax; on the other hand, chylous pleural effusions are not necessarily milky. Though idiopathic chylothoraces do occur, chylothorax is not an end-diagnosis. In many cases, after the diagnosis has been confirmed, further investigation is still needed to clarify the aetiology. This chapter discusses the context in which these effusions should be suspected, describes the other conditions that present with a milky pleural effusion, and suggests how they may be discriminated from chylothoraces. It explores the underlying causes that must be sought and presents a diagnostic approach for uncovering these conditions.