Centralisers for oil well casings comprises (a) a mounting (10) by which the centraliser (4) may be mounted on an outer surface of a casihg (11) and several members (12) carried by the mounting (10) at spaced intervals; characterised in that (b) the members (12) are held by a control device (15) in a collapsed disposition in which the members (12) extend along the casing (11) closely adjacent its outer surface; (c) the control device (15) being so operable that the members (12) move from the collapsed disposition to a deployed position in which the members (12) extend away from the mounting (10) for engagement with an associated borehole.
