"Radar in World War II" was prepared by Guerlac in his capacity as historian of the Radiation laboratory at the Massachusetts Institute of Technology. The Radiation Lab was the prime contractor for the work of the National Defence Research Committee (which was concerned with every aspect of the U.S. wartime radar program). The history, focusing on the years 1940-1945, covers all phases of wartime research, production of radar by American industry, and the military use of radar on the ground, in naval and anti-submarine warfare, and in the air for bombing and navigational uses. This document is the uncut (and now declassified) version of Guerlac's history.