Being the second mode that traditionally is encountered in the study of heat transfer, with the analysis of convection we learn on the heat transfer process that is executed by a fluid stream. The stream purposefully acts as a heat carrier between two moving media in relative motion coming in contact. It is clear, then, that this topic is focussed on the effects of solid/fluid heat interaction. So, while studying convection, we exploit the notions learned with the fluid mechanics that focussed on the effects of solid/fluid dynamic interaction. As we recall that convective heat flux is ruled by the description of a heat transfer coefficient, we describe briefly some basic devices where convection is realized following the microscopic balance leading to join the distributions of the flow velocity vector and the temperature scalar. It is clear that, at this point, we should distinguish between the temperature of the solid \(T_\mathrm {s}\), the one studied in Chap. 2, and the temperature of the fluid \(T_\mathrm {f}\) that will be the subject of this chapter, but whichever temperature we consider, we really face with the same scalar variable. However, for the sake of clarity sometime we indulge in differentiating the nomenclature, but unless we will be engaged in two-phase (solid/fluid) modeling, we will hold that the subject at stake now is the fluid temperature \(T_\mathrm {f}\). Along the same lines that we exploit so far, then we derive and integrate the governing differential equations in various cases, and the concept of the thermal boundary layer is presented that carries analogies with the former fluid boundary layer. Finally, a numerical solution of the governing equations is completed, for the distribution of temperature and velocity, following the course cast so far.