An insertion-locking pin and grommet fastener (20, 120) comprising: a grommet (24, 124) defining a central aperture with at least two adjacent longitudinally extending leg members (30, 130); a pin (31, 131) having a first locking member in the form of one of a recess and a protrusion that engages a first cooperating locking member of the grommet (24, 124) in the form of an opposite one of a recess and a protrusion to retain the pin (31, 131) in a first position relative to the grommet (24, 124) in which the longitudinally extending leg members (30, 130) of the grommet (24, 124) are in a laterally unexpanded state; the grommet (24, 124) and the pin (31, 131) being separate components, respectively, wherein the pin (31,131) can be inserted into the central aperture of the grommet (24, 124); the pin (31, 131) having a second locking member in the form of a second protrusion that engages the at least two adjacent longitudinally extending leg members (30, 130) to retain the pin (31, 131) in a second position relative to the grommet (24, 124) in which the longitudinally extending leg members (30, 130) of the grommet (24, 124) are in a laterally expanded state; characterised in that the pin (31, 131) has a laterally extending driving protrusion (38, 138), wherein during insertion of the pin and grommet into a joining aperture (46, 146) of a joining part, the driving protrusion (38, 138) automatically engages a periphery of the joining aperture (46, 146) to drive the pin (31, 131) from the first position relative to the grommet (24, 124) to the second position relative to the grommet (24, 124) to lock the pin and grommet to the joining part. a grommet (24, 124) defining a central aperture with at least two adjacent longitudinally extending leg members (30, 130); a pin (31, 131) having a first locking member in the form of one of a recess and a protrusion that engages a first cooperating locking member of the grommet (24, 124) in the form of an opposite one of a recess and a protrusion to retain the pin (31, 131) in a first position relative to the grommet (24, 124) in which the longitudinally extending leg members (30, 130) of the grommet (24, 124) are in a laterally unexpanded state; the grommet (24, 124) and the pin (31, 131) being separate components, respectively, wherein the pin (31,131) can be inserted into the central aperture of the grommet (24, 124); the pin (31, 131) having a second locking member in the form of a second protrusion that engages the at least two adjacent longitudinally extending leg members (30, 130) to retain the pin (31, 131) in a second position relative to the grommet (24, 124) in which the longitudinally extending leg members (30, 130) of the grommet (24, 124) are in a laterally expanded state; characterised in that the pin (31, 131) has a laterally extending driving protrusion (38, 138), wherein during insertion of the pin and grommet into a joining aperture (46, 146) of a joining part, the driving protrusion (38, 138) automatically engages a periphery of the joining aperture (46, 146) to drive the pin (31, 131) from the first position relative to the grommet (24, 124) to the second position relative to the grommet (24, 124) to lock the pin and grommet to the joining part.