AbstractIn Roux's modification of gastroenterostomy, the bowel is transected at the duodenojejunal junction. The proximal jejunum is united to the stomach. The duodenum is joined, end‐to‐side, into the jejunum at a distance from the gastrojejunostomy. Jejunal peristalsis is unidirectional, and contents entering its proximal end are removed and passed distally since jejunum is a transmitting—not a capacitance—part of the intestine. The loop can be used to drain away contents and also acts as a one‐way valve, preventing reflux. A single segment of intestine can be extended much farther than an intact loop, while retaining an adequate blood supply. Originally used to prevent intragastric bile reflux, the technique has been exploited to allow esophagojejunal anastomosis and drainage of the biliary and pancreatic ducts. The ability of a Roux‐en‐Y loop of jejunum to remove and pass on contents makes it a valuable means of draining cysts and fistulas. Preparing a Roux‐en‐Y segment demands careful preliminary assessment of the vascular supply to the upper jejunum since the pattern is inconstant. The technique of dividing the arterial and venous arcades to increase the reach of the jejunal segment is described, together with some of the more frequently used modifications of Roux's method.