The combination of thermally grown oxide on silicon has achieved preeminence in technology because of its remarkable properties. As a result of extensive investigation, many of the properties of the interface are well understood. This article reviews the main lines of evidence for our present understanding. The energy‐band relations at the silicon–SiO2 interface have been determined mainly by measurements of internal photoemission. These data can be combined with the results of C–V measurements to give a detailed picture of the behavior of mobile ions in the oxide.