Every flow is characterized by certain macroscopic quantities: the velocity u, the pressure p and the molecular density N, or what amounts to the same, the mass density = Nm. The kinetic theory of gases, the product N. P, also called the distribution function, plays the same role as the angular flux density introduced in neutronics. The fundamental variable v simply replaces the couple (E,). Moreover, the distribution function of the gas satisfies a kinetic equation analogous to the most general neutron transport equation, the latter being only an adaptation of the kinetic equation established a century ago by L. Boltzmann. In a molecular flow the mean free path of the molecules is by definition much larger than the diameter of the conduit under consideration. The interactions with the wall thus play a crucial role. This type of flow was first studied by Knudsen and it is known by his name.