Density functional theory has been used to perform a comparative theoretical study of the adsorption and dissociation of H2O monomers and icelike bilayers on Ru{0001}. H2O monomers bind preferentially at atop sites with an adsorption energy of similar to0.4 eV/H2O. The main bonding interaction is through the H2O 1b(1) molecular orbital which mixes with Ru d(z)(2) states. The lower-lying set of H2O molecules in an intact H2O bilayer bond in a similar fashion; the high-lying H2O molecules, however, do not bond directly with the surface, rather they are held in place through H bonding. The H2O adsorption energy in intact bilayers is -0.6 eV/H2O and we estimate that H bonding accounts for -70% of this. In agreement with Feibelman (Science 2002, 295, 99) we find that a partially dissociated OH + H2O overlayer is energetically favored over pure intact H2O bilayers on the surface. The barrier for the dissociation of a chemisorbed H2O monomer is 0.8 eV, whereas the barrier to dissociate a H2O incorporated in a bilayer is just 0.5 eV.