Hydraulically assisted fastener (10) comprises a body (11) with a central bore (12) to engage a connector element (20) and with an annular recess (14) opening outwards to an end surface (15). Annular thrust member (17) fits into and seals the recess (14). Charging medium (50) is injected under pressure into annular chamber (30) defined by recess (14) and member (17) and moves body (11) relative to member (17) to tension element (20). Medium (50) sets in chamber (30) to maintain the tension in element (20). Element (20) may be a bolt or stud, body (11) can be a nut and member (17) can be a plan washer. Medium (50) can be a curable viscous paste, a suspended solid in a self-setting compound or a particulate solid which behaves as a fluid. Suitable materials include graphite, and lead, copper or steel balls.