Experiments on electroforming of Metal-Oxide-Metal thin film sandwiches which have been electroformed to exhibit voltage-controlled negative resistance are summarized and an outline of recent evidence in favour of localized or filamentary conduction is given.A similar review is given of the experiments on oxide sandwich structures which have been formed to exhibit current-controlled negative resistance or threshold switching and memory switching. Current theories are reviewed briefly. Finally oxide memory devices are compared with those based upon the chalcogenide glasses.