An analytical expression, in terms of the bubble population, was derived for the velocity of the liquid adjacent to the heating surface. This velocity was used to estimate the heat transfer in nucleate boiling. Experimental data indicate that when bubbles can be considered as isolated, then the heat transfer in nucleate boiling is intermittent and bears a similarity to the transfer of heat from a flat plate with boundary layer formation. Experimental data indicate also that as the vapor void fraction (vapor hold-up) on the heating surface increases there occurs a change in the hydrodynamic conditions of the two-phase system. This change occurs at a heat flux density well below the critical heat flux. The vapor removal is altered from an intermittent to a continuous process. The mechanism of heat transfer is affected but not the process.