Most photonics devices consist of junctions of various materials, including interfaces with oxide films that have been grown intentionally or formed unintentionally (e.g., oxidized in air). Although the material volume of oxide interface layers remains limited compared to the total volume of device materials, the interface layers include a high density of electronic defect states around the band gap, which cause issues such as nonradiative surface recombination and leakage currents. Therefore, the interfaces affect the operation of various devices. It is, however, challenging to characterize and modify the interface properties in a controlled manner due to the embedded nature of the interface layers and strong reactions of solid surfaces with the environment (e.g., with oxygen). In this chapter, the challenges and possible solutions concerning the research and development of the oxide interfaces are explored by discussing investigations of the oxide-film interfaces with established semiconductor crystals—namely, silicon (Si), germanium (Ge), and gallium arsenide (GaAs).