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We have calculated the electronic structure of the spin-dependent tunneling structures, Fe|Ge|Fe and Fe|GaAs|Fe, using first principles techniques. We find that there is a large charge transfer from the metal layer to the semiconductor layer; 0.21 electrons are transferred from Fe to Ge and 0.27 electrons are transferred from Fe to GaAs at each interface. The density of states of the interfacial metal layer is dramatically different from the other metal layers; there is a large peak in the density of states at the Fermi energy for the minority electrons. The electronic structure of the semiconductor layer is quite different for the majority and the minority spins although its total magnetic moment is negligible. Our results suggest that the theory of spin-dependent tunneling using the simple model of a potential barrier or a model based on densities of states taken from bulk band structures may not apply to these systems.