A medicament delivery device comprises a housing, a holder within the housing for receiving a medicament cartridge, a piston rod for driving a bung of the medicament cartridge, a drive mechanism including a motor for providing an output drive to the piston rod for delivering the medicament and control means for controlling operation of the device. The device is additionally provided with a bung sensor for sensing when the piston rod is in contact with the bung and the control means is operative for advancing the drive of the piston rod towards the bung.