The DOE and the Federal Bureau of Investigation have long viewed these facilities, especially its nuclear weapons design laboratories and its nuclear material and weapons production facilities, as targets of espionage and other threats. To protect its facilities from security threats, DOE created a multifaceted, defense-in-depth security strategy. Various lines of defense are used to protect classified and sensitive information, nuclear materials, and equipment. Numerous reviews of security have been performed and show serious weaknesses in many of these lines of defense that have lead to losses of classified or sensitive information and technology. Some of these weaknesses have been identified as security-related problems with controlling foreign visitors, protecting classified and sensitive information, maintaining physical security over facilities and property, ensuring the trustworthiness of employees, and accounting for nuclear materials. There are two overall systemic causes of the security problems. First, there has been a longstanding lack of attention and/or priority given to security matters by DOE managers and its contractors. Second, and probably most importantly, there is a serious lack of accountability among DOE and its contractors for their actions. These two causes are interrelated and not easily corrected. Security problems reflect a lack of accountability. The history of security lapses in the nuclear weapons complex show that DOE is not holding its contractors accountable for meeting all of its important responsibilities. Furthermore, DOE leadership is not holding its program managers accountable for making sure contractors do their jobs. 21 footnotes and 1 appendix