The present invention provides a retaining washer for retaining threaded fasteners, such as bolts, that have been inadvertently loosened, in use. The invention finds particular application in gas turbine engines and in aircraft in general where wire locking of threaded fasteners has traditionally been employed. The washer (10) comprises a base member (14) having a plurality of apertures (18) each for receiving the shank of a respective bolt (20) or stud part of a threaded fastener, and engagement means (16) extending from the base member for engagement with respective bolt head or nut parts of the fasteners to retain the said head or nut parts with respect to the washer in the event of inadvertent loosening. The said engagement means is deformable between an open configuration which enables access to the bolt head or nut parts of the fasteners for tightening and/or removal thereof and a closed configuration in which the bolt head or nut part are captively retained with the washer.