In transport theory, a neutron is considered to be a point particle in the sense that it can be described completely by its position and velocity vector. That makes possible to investigate the process of neutron transport, that is, the motion of the neutrons as they stream about the reactor core, frequently scattering off of atomic nuclei and being absorbed, sometimes leaking out of the reactor. In this chapter, we recall the classic definitions of fundamental neutronic quantities in transport theory: neutron angular density, angle dependant flux and angular neutron current. The time rate of change of neutron angular density is described by the transport equation, introduced in the Chap. 7 . The reader can refer to Bell and Glasstone (1970, pp. 3–7).