A wide variety of measurements on silicon grain boundaries shows that the electronic properties of such boundaries are much like those of Si surfaces in all essential respects. Moreover, the properties of ’’clean’’ surfaces and lightly contaminated surfaces can be studied on many crystallographic orientations of the interfaces without the need for vacuum measurements. Such grain boundaries may therefore be regarded as Si-Si interfaces in the sense used in surface physics. It is shown that the thermal history and the dissolved oxygen of the silicon play major roles in determining the interface properties.