The supine and Trendelenburg positions should be avoided or minimized because of the drastic reduction in lung volumes from cephalad displacement of the diaphragm. These positions can also result in increased venous return, cardiac output, pulmonary blood flow and volume, and arterial blood pressure. Elevating the upper body 25–30° in a semi-recumbent or reverse Trendelenburg’s position improves pulmonary mechanics. The prone position is usually well tolerated by obese patients provided the abdomen is not compressed although an extremely large pannus, even if freely hanging (e.g., on a spine fusion or Jackson table), may place undue traction on the mesenteric circulation. The lateral position is typically well tolerated.