This lecture covers the fundamental concepts of neutron physics in nuclear reactors, including the determination of reactor criticality, neutron production, and absorption. Topics include the four-factor formula, neutron fission efficiency, fissile nuclides, neutron distribution, and the slowing down process. The lecture also discusses the neutron balance equation, Fick's Law, diffusion theory, and the Laplacian operator in different geometries. Additionally, it explores the characteristics of moderators, elastic collisions, and the slowing down mechanism in thermal reactors. The criticality condition, eigenvalues, and eigenmodes for spherical and cylindrical reactors are explained, along with the relationship between neutron flux and thermal power in different reactor geometries.