We study the infrared properties of the Si-SiO2 interface within a first-principles approach. In order to provide an atomic-scale description of the dielectric permittivity (both high-frequency and static) and of the infrared absorption at the interface, we introduce two theoretical schemes of general validity. First, we develop a method for investigating atomic-scale dielectric permittivity profiles across interfaces between insulators. From the microscopic charge density induced by an applied electric field, we calculate a local permittivity which describes variations of the dielectric response over length scales of the order of interatomic distances. In order to establish a relation between the dielectric response and the underlying microscopic structure, the local permittivity is further analysed in terms of maximally localized Wannier functions. Second, we develop a method for calculating from first principles both the transverse-optical and longitudinal-optical infrared absorption spectra at surfaces and interfaces. We derive expressions for the total absorption spectra of the system under consideration, and then define a spatial decomposition which provides the evolution of the infrared activity across that system. Such a decomposition is particularly suited for associating specific spectral features to the underlying local bonding arrangements. By using the first method, we determine the profile of the local permittivity across several structural models of the Si-SiO2 interface. We are able to show that the dielectric transition from the silicon to the oxide occurs within a width of only a few angstroms, and that the interfacial layer carries an enhanced permittivity with respect to bulk vitreous silica. Correspondingly, the equivalent oxide thickness of the interfacial oxide is found to be smaller than the corresponding physical thickness by 0.2–0.3 nm, with beneficial consequences for the scaling of Si-based electronic devices. By using the second method, we solve a long-standing controversy related to the red shifts of the high-frequency peaks observed in the infrared spectra of ultrathin oxides on silicon with decreasing thickness. By calculating the transverseoptical and the longitudinal-optical absorption spectra across a realistic model of the Si(100)-SiO2 interface, we are able to assign the microscopic origin of these shifts to the lengthening of the Si-O bonds in the interfacial substoichiometric oxide.