A rope tensioner is used, for example, to connect a rope 1 under tension to a safety switch whereby the switch operates, e.g., to turn off power to machinery guarded or bounded by the rope in the event that the rope tension departs significantly from a predetermined value. The rope tensioner ensures a firm gripping of the rope 1 reducing risk of the rope 1 slipping and losing the required tension. The rope tensioner comprises to this end a housing 2 with passageways 16,17 to receive the rope 1 running between main inlet and outlet locations and a clamp 14 to apply a clamping force transverse to the rope 1 at a clamping location 8 within the passageway 16, 17, the passageway having sections running in opposed directions.