The forced heat transfer phenomenon inside tubes and ducts is analyzed in this chapter. First, hydrodynamic and thermal boundary layer development in tubes is analyzed. The concept of cup or bulk temperature as the effective cross-section temperature for heat transfer calculation is introduced. The Reynolds number for laminar and turbulent regimes is discussed. The laminar case is studied to analytically obtain the Nusselt number for the constant heat flux at the wall and extend it to constant wall temperature. Values for various cross-sections are presented. Heat transfer in the turbulent regime is analyzed in subsequent sections and working expressions are given for many situations of interest and the friction loss coefficient. In the final section, the logarithmic mean temperature difference, \(LMTD\) , concept is presented and illustrated by working examples.