Abstract: There is described a tamper-proof fastener for use in security applications, the fastener comprising inner and outer members interconnected by flexible straps. The inner and outer members each comprise a leading end, a shank, and a trailing end, the shank of the outer member consisting of two substantially parallel legs defining between them an open-sided passage which contracts towards the leading end, and the leading end of the outer member consisting of two limbs which define between them an open-sided space and converge to a tip where they are interconnected, each limb being connected to a respective leg and presenting an external shoulder facing away from the tip and an internal shoulder facing towards the tip, the trailing end of the outer member including outwardly extending portions presenting an external shoulder facing the leading of the outer member.