Design and operation of nuclear reactors require knowledge of the conditions under which a reactor will be critical as well as the degree of sub or supercriticality when these conditions change. In addition, knowledge is required of the spatial distribution of neutron reaction rates in reactor components as a prerequisite, for example, for inferring temperature distributions and thus the satisfaction of thermal limit requirements. Both reaction rate spatial distributions and reactivity can be and have been measured by suitable experimental techniques either in mockups or in the operating reactors themselves. These quantities can also be calculated by various techniques. As nuclear cross sections have become more accurate and as calculational methods have been refined, the tendency has been to rely more heavily on calculations. Available reactor experimental data have been used to validate the calculations within reasonable margins.