The smuggling of nuclear materials, including weapons-grade uranium and plutonium, arose in a particularly problematic form in the early 1990s, and gave rise to the development of a new forensic discipline—nuclear forensics. Its principal aim is to determine the origin of nuclear and other radioactive materials that are out of regulatory control. Existing analytical techniques as used in material science, nuclear safeguards, and environmental analysis have been adapted to the specific needs of nuclear forensic investigations. During the past 20 years, new methodologies have been developed, aiming at identifying useful “nuclear forensic signatures” in order to reduce the ambiguities remaining in the interpretation of analytical results. The hazards involved with nuclear smuggling and the potential relation with nuclear terrorism are the driving forces for deploying and further advancing this scientific area.