READERSHIP: This introduction to reactor kinetics is targeted to those following a specialized course of study in nuclear engineering or those interested in the way fission reactors work. While no knowledge of nuclear physics is necessary for an understanding of the material, mathematical methods covered in the first years of university are a prerequisite. This book should allow the reader to understand kinetics sufficiently to predict the behaviour of a reactor subjected to various perturbations and to perform more detailed calculations. Reactivity is a reactor-physics concept that allows us to characterize in a simple fashion the dynamic state of a reactor. Its definition stems from point kinetics, and an effort was made to introduce it in a general and rigorous way. Since the concept of reactivity is widely used in practice in reactor simulation and control, nuclear-station operations personnel will be able to make good use of the presentation. By recalling the fundamental theory and the more basic methods available, this book can also serve as a reference work for engineers and analysts engaged in technical-support activities for reactor operation. NEED: The theory presented has a general scope and applies to all reactor types. However, the material is illustrated with numerical examples characteristic of the behaviour of heavy-water reactors of the CANDU type, since these are in general neglected in manuals. Their response is compared to that of light-water reactors. SUBJECT: The material in this introductory work is centred around point kinetics. We are interested mostly in the evolution of the total power of a reactor, in response to various perturbations that may be applied. However, point kinetics allows the estimation of the change in total power only via perturbations to average reactor properties. Since perturbations are generally localized (i.e., they have an effect on only part of the domain), the weighting procedure for determining changes in the average properties is central to the point-kinetics approach. This weighting must take into account the diffusion of neutrons and the non-uniform distribution of materials in the core. It is therefore essential to derive first the point-kinetics equations from the more general equations of space-time kinetics.