The present invention relates to a downhole tool (102) for selectively opening and closing a lateral borehole extending from a primary borehole. The tool (102) comprises a body (104) incorporating a wall provided with an opening (116) extending therethrough; a deflector member (106) slidably mounted in the body (104) and having a deflecting surface (126) for deflecting, in use, downhole equipment laterally within the body (104). The deflector member (106) is slidable between an open position, in which the deflecting surface is located adjacent and facing towards said opening so that, in use, downhole equipment laterally deflected by the deflecting surface is directed through said opening, and a closed position, in which the deflecting surface is oriented relative to said opening so as to prevent downhole equipment from being laterally deflected through said opening during use. In either position, equipment may pass into the primary borehole downhole of the primary/lateral junction. The tool (102) also comprises constraining means (138, 140) for restricting movement of the whipstock relative to the body.