Adequate representation of the interactions that take place between water molecules has long been a goal of force field design. A full understanding of how the molecular charge distribution of water is altered by adjacent water molecules and by the hydrogen-bonding environment is a vital step toward achieving this task. For this purpose we generated ab initio electron densities of pure water clusters and hydrated serine and tyrosine. Quantum chemical topology enabled the study of a well-defined water molecule inside these clusters, by means of its volume, energy, and multipole moments. Intra- and intermolecular charge transfer was monitored and related to the polarization of water in hydrogen-bonded networks. Our analysis affords a way to define different types of water molecules in clusters. © 2007 American Chemical Society.