The institution of one-lung ventilation (OLV) can be problematic in approximately 20% of any case mix. A significant number will prove difficult at the lung separator insertion stage. The usual conditions of difficult intubation pertain, as well as some specific to the thoracic discipline and the pathologies encountered and exacerbated by the structure and bulk of some lung separators. The use of OLV and the adoption of the lateral decubitus position results in specific physiological changes, such as the shifts in west zones from vertical to horizontal, which are best countered by positive pressure ventilation. The open pneumothorax, weight of mediastinum, abdominal contents on adoption of the lateral decubitus position and the surgeon at work compress the dependent lung; and, all must be opposed through the narrow conduit of, often, the single lumen of a double lumen endobronchial tube (DLT).