A sash window restrictor (20) has a housing (22) which is to be let into a vertical frame member (10) of one sash (normally the upper, outer sash) of a window, at a position, say, 10 cm above the top rail of the lower sash when the windows are closed. A tab (24) is mounted in the housing and can be retracted into the housing or extended from the housing. A mechanism acts between the tab and the housing and allows the tab to be pressed into the housing and held in the housing for a period of time before the tab is released to allow the tab to extend. With the tab extended, the lower sash (12) can be lifted until it reaches the tab (24) but no further, so that a 10 cm gap will be opened at the bottom of the window.