The Manhattan Project culminated in the design and fabrication of two types of nuclear weapons–Little Boy and Fat Man. The first type was exploded over Hiroshima, the second over Nagasaki. Estimates of radiation exposures depend in part on explosive yields, and much of the evaluation of radiation effects upon man depends on data from the Hiroshima and Nagasaki explosions. The yield of the Fat Man has been determined rather well, being given variously from 19-24 kt. (Present official yield is 23 kt.) Estimates for the Little Boy have ranged from 6-23 kt. (The current official yield is 13 kt.) The data from which estimates may be made are fragmentary, and the parameters needed for evaluation have been either missing, inconsistent, or erroneous. Part of the problem arises from President Truman-s edict that the yields of both explosions were 20,000 tons of TNT; another part of the problem arises from the inadequate and faulty documentation of the combat missions by the Army Air Corps 509th Composite Group and the US Air Force historians. Many well-researched books are useful in resolving inconsistencies. There is also documentation by the US Strategic Bombing Survey (USSBS), the Manhattan Engineering District, and the Japanese. These sources are not particularly useful for yield evaluation; however, they contain clues.