A needle safety shield assembly for an injector comprises: a needle shield (231) for shielding a needle of the injector and a shield locking means comprising a track (233 236) adapted to engage with a runner (224) the track comprising at least one unidirectional barrier (232 237) through which the runner can pass in which movement of the needle shield to expose the needle for use and to subsequently re shield the needle causes movement of the runner (224) along the track (233 236) through the at least one unidirectional barrier (232 237) and into a locked position in which further movement of the runner and needle shield is prevented.
