This chapter discusses the human side of physical security, identifying and evaluating common practices used to mitigate insider threats, including personnel reliability programs, access controls, and defensive forces. Indian author Rajeswari Rajagopalan argues that insiders pose some of the most serious threats to the critical infrastructure of the nuclear enterprise. She points out that complacency can allow insider threats to take root and identifies two areas of improvement for personnel reliability programs: more thorough vetting of temporary workers and persistent monitoring of employees’ online activities. U.S. authors Todd Burbach and Patrick Lynch agree that insiders can pose significant threats to nuclear safety and security. They highlight the role of trustworthiness programs, which rely heavily on workforce cooperation, in mitigating insider threats.