Keeping the Nation's Secrets: A Report to the Secretary of Defense By the Commission to Review DOD Security Policy and Practices [open pdf - 167KB]
"Each year thousands of classified programs and projects are carried out by the Department of Defense, through its components and its contractual base, in a wide variety of operational and geographical settings. These activities generate millions of items of classified information, ultimately disseminated to almost four million individuals who require such information to perform their assigned tasks. This classified information is not only in the form of documents. An enormous inventory of classified equipment, both end items and components, must be safeguarded; and, increasingly, classified data is being processed, transmitted and stored electronically, posing serious new problems of protection. Arrayed against this vast and immensely important target are the intelligence services of the Soviet Union, its surrogates and other countries with interests hostile to the United States and its allies. In combination, those services conduct massive and highly organized collection operations to acquire all information, classified and unclassified, of military value. Although a variety of means, both human and technical, are employed, human collection constitutes the more significant threat within the continental United States today."
Federation of American Scientists: http://www.fas.org/main/home.jsp