Since the beginning of the 1990s when the first seizures of nuclear material were reported, the IAEA has recorded over 200 cases of illicit trafficking of nuclear materials. Despite the decreasing frequency of nuclear material seizures, particularly the ones involving weapons-grade material, the issue continues to attract public attention and is a reason for concern due to the hazard associated with such materials. Once illicitly trafficked nuclear material has been intercepted, the questions of its intended use and origin are to be addressed. Especially the origin is of prime importance in order to close the gaps and improve the physical protection at the sites where the theft or diversion occurred. To answer these questions, a dedicated nuclear forensics methodology has been developed. In this paper an overview is given on the methodologies used, on the past and on-going developments and on the experience gathered. Some selected examples shall illustrate the challenges and the complexity associated with this work.