The U.S. Nuclear Regulatory Commission (NRC) is issuing Revision 1 to Regulatory Guide (RG) 5.77, "Insider Mitigation Program," to provide licensees and applicants with agency approved guidance for complying with NRC regulations. RG 5.77 applies to nuclear power reactors that contain protected or vital areas. Licensees should use defense-in-depth methodologies to minimize the potential for an insider to adversely affect, either directly or indirectly, the licensee's capability to prevent significant core damage or spent fuel sabotage.