There are concerns that terrorist or criminal groups could gain access to high activity radioactive sources and use these sources maliciously. The IAEA is working with Member States to increase control, accounting and security of radioactive sources to prevent their malicious use and the associated potential consequences. Based on extensive input from technical and legal experts, this Implementing Guide sets forth guidance on the security of sources and will serve as a useful tool for legislators and regulators, physical protection specialists, and facility and transport operators, as well as for law enforcement officers.