Abstract: A security device for a video recorder or player is adapted for insertion into the cassette-receiving opening and for locking therein.The device has a body (1) housing a locking mechanism (6) which cooperates with opposed locking members (4a, 4b) which are movable between retracted positions and the illustrated extended positions in which the members can frictionally engage the respective internal side walls of the opening to retain the device. The dimensions of the body are substantially the same as a standard video cassette except that the depth c is smaller; this and the fact that the device has a peripheral flange (2) adapted to engage the periphery of the opening thereby covering the opening and preventing the device from being inserted too far into the opening, prevent actuation of the cassette-receiving mechanism when the device is inserted in the cassette-receiving opening.