The electronic structure and bonding at different oxygen sites of MoO3(010) and (100) surfaces is reviewed on the basis of ab initio density functional theory (DFT-LCGTO) cluster calculations. The clusters are chosen as finite sections of the ideal MoO3 surface where cluster embedding is achieved by bond saturation with hydrogen terminator atoms yielding clusters up to Mo7O30H18. Resulting charge density distributions and binding properties are analyzed by populations, bond orders, and electrostatic potential maps. Interatomic binding at the surface is determined by both ionic and covalent contributions with a clear distinction between terminal oxygens and different bridging surface oxygens. Electronic differences between the MoO3 (010) and (100) surfaces are found to be mainly due to the different atom arrangement while local atom charging and binding properties seem surface independent. The electronic surface parameters influence the behavior and reactions of adsorbed molecules as will be shown for H, OH, and C3H5 adsorbates.