The second argument concerning transport phenomena relates to energy. We begin with the treatment of thermal conduction, whereby the general law of molecular transport is transformed into the Fourier’s equation. The introduction of the concept of thermal conduction allows us to define the Prandtl number, seen as the ratio between the molecular transport of momentum and that of energy. We then continue with thermal convection, which we apply to the case of a fluid animated by fully developed turbulent motion. We use the film theory to concentrate the resistances to energy transfer in a boundary layer adjacent to the wall, and we define the Nusselt number as the ratio between the total transport and the molecular transport of energy. The analysis of some cases (energy transport through flat or cylindrical walls) allows us to highlight how the heat flux can be expressed in terms of the ratio between driving force and resistance, or of the product between driving force and heat transfer coefficient (the latter being the reciprocal of the resistance), and how the overall resistance to energy transport can be expressed as the sum of the resistances to the individual transport phenomena placed in series.