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.
