Data concerning the existence, size, and significance of an anti-Nazi opposition within Germany are forthcoming from two primary sources. The first source is the direct testimony of opposition leaders still surviving after the occupation; the second source consists of official German intelligence reports or interrogations of interned Gestapo and Sicherheitsdienst officials. The direct testimony of opposition leaders is, of course, subject to the qualification that it is to the interest of the leader and his group to represent the activities of his movement during the war years in the best possible light. Estimates of the size and scope of activities provided by such leaders may be viewed as more or less exaggerated. However the experience of Bombing Survey Field Teams also suggests that such estimates may in some cases be low rather than high because of the extreme secrecy in which such movements were forced to operate. For example, a number of Communist leaders knew in general terms that other Communist groups and cells were operating in their area, but because of the absence of any connection they were unable to estimate the size of the group. Throughout the opposition movement it was an elementary principle of safety, confirmed by repeated experience with Gestapo terror and torture, never to know more about the personnel and activities of the movement than was absolutely essential. In evaluating the information from this source it is also necessary to keep in mind that the best informants in a great many cases had been executed in the last wave of terror. Frequently the knowledge of the survivors was fragmentary; many of those who had occupied central points in the organization had fallen.