A molecular picture of hydrophilic and hydrophobic interactions, which ubiquitously exist in nature, has been proposed based on ab initio density functional study of water at two prototype metal (Pt and Au) surfaces. We demonstrate that the hydrophilicity-hydrophobicity can be characterized by the water-surface coupling and the strength of the hydrogen bond at the interfaces. From this picture, Pt is found to be hydrophilic while Au is hydrophobic, in agreement with experiment. The effect of the charge transfer and the long-ranged electron polarization of water on these interactions are also elaborated.(C) 2003 American Institute of Physics.