  The physical nature and the correct definition of hydrogen bond (H-bond) are
considered.\,\,The influence of H-bonds on the thermodynamic, kinetic, and
spectroscopic properties of water is analyzed.\,\,The conventional model of
H-bonds as sharply directed and saturated bridges between water molecules is
incompatible with the behavior of the specific volume, evaporation heat, and
self-diffusion and kinematic shear viscosity coefficients of water. On the
other hand, it is shown that the variation of the dipole moment of a water
molecule and the frequency shift of valence vibrations of a hydroxyl group can
be totally explained in the framework of the electrostatic model of
H-bond.\,\,At the same time, the temperature dependences of the heat capacity
of water in the liquid and vapor states clearly testify to the existence of
weak H-bonds.\,\,The analysis of a water dimer shows that the contribution of
weak H-bonds to its ground state energy is approximately 4--5 times lower in
comparison with the energy of electrostatic interaction between water
molecules.\,\,A conclusion is made that H-bonds have the same nature in all
other cases where they occur.
