Plutonium fits near the middle of the actinide series. It is of practical interest principally because the 239 isotope has attractive nuclear properties for energy production and nuclear explosives. Its structural properties are particularly unusual. The thermal instability of plutonium—that is, the large length (or volume) changes during heating or cooling—is the most important consequence of the unusual properties of plutonium. The huge volume changes in the solid state result primarily from structural transformations among an unprecedented six solid allotropes. In addition, plutonium’s continuous radioactive decay causes self-irradiation damage that can fundamentally change its properties over time. ©2000 American Institute of Physics.