This lecture covers the utilization of radionuclide properties to determine origin, age, used, and other properties of nuclear material. These isotope properties are defined as signatures of nuclear material, and include concentrations, relative amounts, and ratios. Specific details are provided on forensic signatures related to Pu and actinide isotopics. The signatures arising from reactors, separations, and post-detonation are included. For plutonium production these signatures include reactor power, reactor type, time of irradiation, separation method, and time since separation. The signatures include plutonium isotopic mass ratios, plutonium isotopic activity ratios, and transplutonium isotope ratios. Separation signatures include evaluation of Zr, Tc, Ru, and the lanthanides, with examples using Nd isotopics. Alloys of actinide metals as signatures is also presented.