A fall arrest device comprises a guide (3) for a track member (7) which, in use, passes through the device, a cam member (9) including a cam portion, and a clutch (21) responsive to at least one of the relative speed and relative acceleration of the fall arrest device and the track member. The clutch serves to determine when the relative speed and/or acceleration is at least a predetermined value. The clutch includes a trigger arm (19) connecting to the cam member so as to move the cam portion of the cam member toward the guide so as to lock the track member between the cam member and the guide when at least the predetermined value is attained.