A window fan which can be mounted in a window opening in either a double-hung window frame or a vertical slider window frame and which can be held securely in place without the use of additional hardware or mounting brackets. The window fan comprises a housing of generally rectangular shape, the housing including a front piece, a back piece, and a pair of side extensions. The side extensions have outer edges sized and shaped to fit within the left and right side railings of a double-hung window or the top and bottom railings of a vertical slider window. Each one of the side extensions includes a pair of outer guides and a plurality of panels telescopically mounted within the outer guides. The side extensions extend out from the sides of the housing at about the rear edge so as to permit installation in windows having outer screens. A plurality of locking mechanisms are provided which, when in the closed position, keep the extenders locked at a desired extension and which, when in the open position, permit a ratchet-type extension or compression of the extenders. A pair of fan units are disposed within the housing in front of the back piece, the fan units having electrically reversible motors which are independently controllable so that either both fans can be used to intake air or to exhaust air or so that one fan unit be used to intake air while the other fan unit is being used to exhaust air.