In this paper different methods for characterizing the defect properties of silicon dioxide and its interface to silicon in metal oxide semiconductor devices, and the resulting achievements in the field of reliability assurance are reviewed. Methods are discussed which allow sensitive characterization of defects both at the interface and in the oxide. Different electrical characterization and injection methods, the charge pumping and gated diode methods are presented. Those methods have led to a well-accepted classification of the defects into interface traps of acceptor-and donor-type and fixed charges in the oxide. Subsequently, the microscopic spin-dependent recombination method is discussed which alone is able to give information on the atomic and molecular level of understanding. Finally, reliability assurance methods are presented, connecting hot-carrier degradation data of electrical parameters with the operation in digital and analog circuits. This leads to the definition of lifetime criteria useful in practice.