Linear augmented-plane-wave calculations of the electronic and structural properties of CaSi2 for three types of stacking of Ca layers and Si double layers are presented. The calculated structural parameters for the two experimentally observed structures are in excellent agreement with experiment. The three different structures contain Ca with sixfold, sevenfold, and eightfold coordination, respectively, with Si. The cohesive energies of the three structures are found to be almost identical and their band structures only weakly dependent on the stacking sequence (or, equivalently, Ca coordination). Although d states are not occupied in the Ca atom, these states are found to hybridize strongly with Si p states near the Fermi level. The Fermi surface is found to consist of Ca d–like electrons and Si p–Ca d–hybridized holes. Consequences of these results for the CaSi2/Si(111) interface are discussed.