We use tight-binding theory to investigate the electronic contribution to dielectric susceptibility in thin films and interfaces of covalent materials. We begin by describing the effects of an electric field on the elemental unit of a covalent material, the bond. Then, we show how the responses of individual bonds can be added up to obtain an estimate of the susceptibility of a bulk material. In doing so, we see that the polarization of a material can be viewed as arising from the transfer of charge from one side of the system to the other, and that this viewpoint leads naturally to a local definition of susceptibility in semiconductors. Using this concept, we examine dielectric susceptibility in thin films and interfaces, with a Si/Ge/Si heterostructure serving as an example. The interesting feature of thin films and interfaces is that they exhibit spatial variations in susceptibility, which we attribute to: (i) elastic distortions; (ii) the creation of bonds at an interface which are of a type not found in either bulk material; and (iii) the coupling of a bond to neighboring antibonds different than those in the bulk material. We then ask what error is introduced by neglecting these local variations when calculating the capacitance of a multilayer dielectric. For the Si/Ge/Si heterostructure, we find that effect (iii) introduces only small errors, even for very thin Ge layers, because the decrease in susceptibility on the Ge side of an interface is offset by the increase in susceptibility on the Si side. Similarly, effect (ii) is small because the polarizability of the Si–Ge bonds at the interface is very nearly the average of that for Si and Ge. On the other hand, effect (i) does lead to noticeable errors, but these errors can be removed almost entirely by choosing the permittivity of the Ge layer to be that of bulk Ge under the same state of strain as the Ge layer in the heterostructure. We conclude by interpreting recent experiments on “high-k” dielectrics in term of what we have learned here. [C. M. Perkins et al., Appl. Phys. Lett. 78, 2357 (2001); M. Koyama et al., Tech. Dig. Int. Electron Devices Meet., 459 (2001); W.-J. Qi et al., ibid., 145 (1999)]. © 2002 American Institute of Physics.