Heat transfer is the flow of thermal energy driven by thermal non-equilibrium (i.e. the effect of a nonuniform temperature field), commonly measured as a heat flux (vector), i.e. the heat flow per unit time (and usually unit normal area) at a control surface. Heat transfer and mass transfer are kinetic processes that may occur and be studied separately or jointly. Studying them apart is simpler, but both processes are modelled by similar mathematical equations in the case of diffusion and convection (there is no mass-transfer similarity to heat radiation), and it is thus more efficient to consider them jointly. Besides, heat and mass transfer must be jointly considered in some cases like evaporative cooling and ablation. The book presents a comprehensive exposure to the theory, definitions, formulae and expenses which are well supported by the prosperity of diagrams and problems in order to make the underlying principles more comprehensive.