This chapter discusses the basic principles of reactor physics. It is difficult to understand the need for many components of a nuclear power plant without some appreciation of the way in which the release of nuclear energy is achieved. The source of the heat that is produced in a nuclear power plant is fission. The only naturally occurring isotope that is capable of a self-sustaining fission reaction is uranium-235. In addition to the fission reaction, there are three other types of reactions that are important. The first type is elastic scattering, in which the neutron simply bounces off the nucleus. Total kinetic energy is conserved, which means that the neutron loses kinetic energy. All isotopes exhibit this form of scattering; therefore, the neutrons that escape any other type of reaction steadily slow down. The second type of reaction is inelastic scattering, which is significant in U238 at energies over about 0.1 MeV. The neutron is absorbed to form a U239 nucleus, which immediately reverts to U238 giving out a neutron and a γ-ray. The net effect is that much of the kinetic energy of the neutron is lost to the γ-ray. The third type of reaction is capture, where an isotope of one higher mass number is formed.