At high reduced pressures extremely high nucleate boiling heat transfer coefficients (HTC) were measured. A single mechanism, which presents a consistent explanation of such HTCs, is very high intensity of liquid evaporation at the periphery of dry spots (nucleation sites) at the heated wall. Due to very small size the nucleation sites can be considered as point heat sinks. Between them convective heat transfer occurs, which in its turn is governed by the inherent mechanisms of boiling. The above two mechanisms comprise a total heat flux from the heated wall in nucleate boiling. The predicting equation, which determines heat flux in boiling via the wall superheat and liquid properties, has been developed with accuracy to two universal numerical factors fitted to the experimental data. Although the equation developed is found to be in good agreement with numerous experimental data for different liquids and in the wide range of reduced pressures and heat fluxes there exists a problem in nucleate boiling, which has not been understood to the full even qualitatively. This problem is the dependence of nucleation site density on the physical properties of the liquid and on the controlling parameters. Some new experimental results by Theofanous et al. [T.G. Theofanous, T.N. Dinh, J.P. Tu, A.T. Dinh, The boiling crisis phenomenon. Part 1: Nucleation and nucleate boiling heat transfer, Exp. Therm. Fluid Sci. 26 (2002) 775-792; T.G. Theofanous, T.N. Dinh, J.P. Tu, A.T. Dinh, The boiling crisis phenomenon. Part II: Dryout dynamics and burnout, Exp. Therm. Fluid Sci. 26 (2002) 793-810.] and Qi et al. [Y. Qi. J.F. Klausner, R. Mei, Role of surface structure in heterogeneous nucleation, Int. J. Heat Mass Transfer 47 (2004) 3097-3107; Y. Qi. J.F. Klausner, Heterogeneous nucleation with artificial cavities. J. Heat Transfer 127 (2005) 1189-1196: Y. Qi, J.F. Klausner, Comparison of nucleation site density for pool boiling and gas nucleation, J. Heat Transfer 128 (2006) 13-20.] require revising the traditional views on a nature of the active nucleation sites in boiling. These results remind the old question: why can the nucleation sites arise at low superheats of the absolutely wettable surface? Obtaining theoretical equation for nucleation site density remains the most significant challenge in nucleate boiling theory. (C) 2009 Elsevier Ltd. All rights reserved.