Highly practical methods for reactivity calculations are discussed in the last section of this chapter. In the design of power reactors or operation of research reactors, it is often necessary to evaluate the effect of small changes (perturbations) on the behavior of the reactors. A local change of reactivity in power reactors, insertion of irradiation pieces in research reactors, and so on should be treated as a local perturbation. In principle, the reactivity change due to a perturbation can be evaluated by repeatedly performing the two- or three-dimensional multi-group calculation. For a small perturbation, however, the reactivity change may be lost in the computations as the result of round-off errors. Enormous calculations are required to obtain the distribution of reactivity coefficients in the core. Such problems can be handled by perturbation theory.