Abstract: A centralizer 10 for mounting on a tubular member such as a section of casing, together with one or more stops 42, 48 for retaining the centralizer thereon. The stops 42, 48 are provided with profiles for engaging the centralizer to restrict rotation thereof. The centralizer 10 may be selectively permitted or restricted from rotation by selection of appropriate stops, and by relative movement of the stops and centralizer on the tubular. Certain embodiments of the centralizer 10 may also include blades 14 on the body thereof, the blades being formed such that the velocity and kinetic energy of fluid flow across the blades is altered, so reducing settling of drill cuttings within the bore and on the centralizer.