The short-term time dependence of two fission products, \( ^{149}{\text {Sm}}\) and \( ^{135}{\text {Xe}}\) , which also have strong absorption cross sections of thermal neutrons, introduces some interesting reactivity change during reactor power transients. The xenon’s behaviour during power changes makes it specially important and interesting. This chapter illustrates the dynamics of xenon and samarium poisoning with the examples of a PWR and a high flux experimental reactor. The phenomenon of xenon induced spatial oscillations is not studied in this book, and the reader will find an extensive bibliography on this subject.