This paper attempts to provide a unified view on electrochromism among the oxides. It is stressed that crystals of all of the well known electrochromic metal (Me) oxides are composed of MeO6 octahedral in various comer-sharing and edge-sharing arrangements, and that these units prevail also in thin films. The coordination of the ions leads to electronic bands that are able to explain the presence or absence of cathodic and anodic electrochromism in numerous defect perovskites, rutiles and layer structures. Optical absorption can be due to intraband effects, interband transitions and polaron hopping.