XU was the largest and most important intelligence gathering organisation in occupied Norway during WWII. Most of the work of XU was kept secret until 1988.

XU was established in July 1940 by Arvid Storsveen, and was originally based on recruiting of students from the University of Oslo. The group connected to professionals around Norway, within railroads, police and so on, and collected maps and photos of German fortifications and forces.

After a period of cooperation with Milorg, XU worked separately from the rest of the Norwegian resistance movement from the autumn of 1941. This step was taken to increase the security.

Having established a network over the entire southern Norway, Gestapo became aware of the organization in the spring of 1942. Several members of the leadership in Oslo, including Storsveen, fled to neutral Sweden in July 1942, and led the organization from abroad. They still had occasional trips to occupied territory, and Storsveen was killed in a Gestapo operation in Oslo in April of 1943. He did not reveal who he was, however, and the organization continued its work under leadership of Øistein Strømnæs.