The unique characteristics of the outer d electrons lead to an extraordinary range of structural and electronic properties for transition-metal oxides. The interface between two transition-metal oxides is of significant interest in electronics and spintronics. Effort has been focused on the growth of atomically sharp interfaces between these oxides. However, an atomically sharp interface does not necessarily imply electronic abruptness. It is important to investigate the electronic transition at these oxide interfaces. In this talk, I will present our work on two types of oxide-oxide interfaces. For one type of interface, antiferromagnetic and insulating NiO and CoO thin films were grown on ferromagnetic and metallic Fe3O4 single-crystal substrates by molecular beam epitaxy (MBE); for the other type of interface, perovskite EuTiO3 thin films were grown on perovskite SrTiO3 single-crystal substrates by pulsed laser deposition (PLD). Electronic states at the buried interfaces were probed either by photoemission
spectroscopy (PES) or by electron energy loss spectroscopy (EELS) based on Scanning Transmission Electron Microscopy (STEM).