An injector device comprising an elongate housing having a proximal end and a distal end, and configured to receive a container of medicament, a needle sleeve mounted within the housing and moveable between an extended position in which the needle sleeve at least partially extends from the distal end of the housing, and a retracted position in which the needle sleeve is received further within the housing than in the extended position, a piston rod moveable longitudinally within the housing, a piston spring configured to bias the piston rod towards a distal end of the housing to engage a container of medicament when received within the housing, and a release mechanism configured to control actuation of the piston rod. The release mechanism comprises a locking arm moveable between a locked position in which the locking arm is in cooperating engagement with the piston rod and prevents movement of the piston rod, and a release position in which the locking arm is out of engagement with the piston rod so that the piston rod is free to move longitudinally within the housing. The needle sleeve is configured such that movement of the needle sleeve from the extended position to the retracted position causes the locking arm to move from the locked position to the release position.