A fastener comprising: a fastener body (16, 116, 216, 316, 416, 516, 616, 716, 816, 916) orientated about a central axis (x-x); said fastener body having a tool-engaging portion (17, 117, 217, 317, 417, 517, 617, 717, 817, 917) to which a driving torque may be applied and a threaded fastening portion (18, 118, 218, 318, 418, 518, 618, 718, 818, 918) configured and arranged to mate with a corresponding threaded element; said fastener body having a shroud-receiving body portion (19, 119, 219, 319, 419, 519, 619, 719, 819, 919) orientated about said central axis, characterised in that a shroud (20, 120, 220, 320, 420, 520, 620, 720, 820, 920) concentrically mounted on said shroud-receiving body portion and having an inner surface facing an outer surface of said shroud-receiving body portion; said shroud being supported in rotatable relationship with said shroud-receiving body portion such that said shroud will rotate relative to said fastener body under an applied external torque prior to said fastener body rotating when said fastener body is engaged with an external structure at a design installation torque; said inner surface of said shroud comprising an annular groove (40, 140, 240, 340, 440, 540, 640, 740, 840, 940) orientated transverse to said central axis; said shroud-receiving body portion comprising a substantially arch-shaped axially-buckled radially-extending annular protrusion (43, 143, 243, 343, 443, 543, 643, 743, 843, 943) extending outwardly transverse to said central axis and having at least an annular apex (44, 144, 244, 344, 444, 544, 644, 744, 844, 944) disposed in said annular groove of said shroud; and said substantially arch-shaped axially-buckled radially-extending annular protrusion of said shroud-receiving body portion and said annular groove of said shroud forming a shroud-retaining element restraining said shroud from movement in at least a first axial direction along said central axis. a fastener body (16, 116, 216, 316, 416, 516, 616, 716, 816, 916) orientated about a central axis (x-x); said fastener body having a tool-engaging portion (17, 117, 217, 317, 417, 517, 617, 717, 817, 917) to which a driving torque may be applied and a threaded fastening portion (18, 118, 218, 318, 418, 518, 618, 718, 818, 918) configured and arranged to mate with a corresponding threaded element; said fastener body having a shroud-receiving body portion (19, 119, 219, 319, 419, 519, 619, 719, 819, 919) orientated about said central axis, characterised in that a shroud (20, 120, 220, 320, 420, 520, 620, 720, 820, 920) concentrically mounted on said shroud-receiving body portion and having an inner surface facing an outer surface of said shroud-receiving body portion; said shroud being supported in rotatable relationship with said shroud-receiving body portion such that said shroud will rotate relative to said fastener body under an applied external torque prior to said fastener body rotating when said fastener body is engaged with an external structure at a design installation torque; said inner surface of said shroud comprising an annular groove (40, 140, 240, 340, 440, 540, 640, 740, 840, 940) orientated transverse to said central axis; said shroud-receiving body portion comprising a substantially arch-shaped axially-buckled radially-extending annular protrusion (43, 143, 243, 343, 443, 543, 643, 743, 843, 943) extending outwardly transverse to said central axis and having at least an annular apex (44, 144, 244, 344, 444, 544, 644, 744, 844, 944) disposed in said annular groove of said shroud; and said substantially arch-shaped axially-buckled radially-extending annular protrusion of said shroud-receiving body portion and said annular groove of said shroud forming a shroud-retaining element restraining said shroud from movement in at least a first axial direction along said central axis.