Neutron flux plays a significant role in the theory of reactor physics, while neutron flux is indirectly measured through the interaction between the neutron and the material used as the neutron detector in experiments. The physical quantity that is measured throughout the experiment is the “reaction rate.” To obtain the neutron flux from the measured reaction rate, various information involving cross sections of the material used as the detector is necessary, in addition to experimental devices. At the same time, it is necessary to consider the theory of reactor physics related to the specific reactions between neutrons and materials in the irradiation field. This chapter introduces methodologies for measuring the reaction rate by the activation foil method before determining the neutron flux.