This report explores the characteristics of 62 reported "insider" crimes that may provide insights into potential threats to the security of Department of Energy (DOE) nuclear weapons programs. In particular, it considers (1) the identity of the insiders, including their motivations, age, length of employment, and status within the corporation, institution, or government agency; (2) the particulars of the crimes, including the illegal actions perpetrated and, in the case of theft, what was taken, as well as how the insiders gained access to their target, and how they were persuaded to commit the crime; and (3) the effectiveness of the security procedures of the corporation, institution, or agency — specifically those implemented to prevent insider crime — and any changes in those procedures that might have resulted from these crimes. The study defines three types of insider crimes: those committed by insiders conspiring with outsiders, those committed by insiders conspiring with other insiders, and those committed by lone insiders. The authors find that the success of most of the incidents examined depended less on detailed planning or expert execution than on the exploitation of existing security flaws. Guard forces are a particular problem, as they were responsible for 41 percent of the crimes committed against guarded targets.