Description of nuclear reactors in general. Use of neutrons: energy production, measurement technology. Cross sections, relationship of nuclear reactions to neutron gas. Description of neutron gas, the Boltzmann equation as a neutron balance. Initial and boundary conditions. The concept of criticality. Approximations of the Boltzmann equation: the diffusion theory. Energy groups. Time dependence and criticality in a one-group approximation. Kinetics, measurement of reactivity. The neutron spectrum. Theory of neutron slowing down. Resonances. Thermalization. Spatial distribution of slowing down neutrons, Fermi age. Multi-group methods. Thermal reactors: bare and reflected reactor. Fuel grids. Reactivity factors. Reactor excursion. Adjoint function. Perturbation theory. Point kinetics. Burn-up. Reactor operation. Numerical methods