Review on neutron interactions, center of mass system, differential cross-section, compound nucleus formation model, optical model, theory of total neutron cross-section, Doppler broadening, neutron current and flux, neutron transport theory, one-speed neutron diffusion theory, neutron moderation in hydrogenous and non-hydrogenous medium, slowing down density , conditions for criticality of nuclear reactors, Four and six-factor formula, neutrons economy, infinite reactor, reflector saving, and boundary conditions.