This lecture covers the concepts of slowing down equations, lethargy, and Fermi age theory in the context of neutron moderation. It discusses the interaction of neutrons with matter, nuclear fission, simplified modeling of multiplying media, and the diffusion of neutrons. The lecture also explores neutron kinetics, reactor technology, and the Fermi age theory for neutron balance in a volume. The physical significance of Fermi age and space-dependent slowing down are explained, along with the relationship between neutron migration and distance from the source.