This publication provides the rationale for the development of a national nuclear forensics library and addresses how a State may use such a national system in investigations of nuclear and other radioactive material out of regulatory control. Nuclear forensics is the examination of nuclear or other radioactive material, or of evidence that is contaminated with radionuclides, in the context of legal proceedings under international or national law related to nuclear security. The analysis of nuclear or other radioactive material seeks to identify what the materials are, how, when and where the materials were made, and what their intended uses were. Important when conducting a nuclear forensics examination is the ability of States to identify nuclear and other radioactive material within the State and determine whether those materials are consistent with domestic holdings. As a system for the identification of nuclear or other radioactive material, a national nuclear forensics library can facilitate interpretation of findings and assist States in this determination. This publication seeks to assist States that choose to develop such a library tailored to their individual circumstances, national legal requirements, and security needs.