A drug delivery device (1) includes a delivery unit, an electronic control system and a plunger sensing system (70). The delivery unit (3) includes a drug container (6) comprising a barrel portion (6a) and a plunger (12) slidably mounted within the barrel portion and sealing against an inner surface of the barrel portion (6a) for containing a liquid drug (78) within the drug container. The plunger sensing system (70) includes an optical sensor (71) with a transmitter (71a) and a receiver (71b), the transmitter configured for transmitting an optical signal to a back end (73) of the plunger (12) and the receiver configured to receive the optical signal reflected off the plunger back end (73). The plunger sensing system is connected to the electronic control system (47) configured to measure a time of flight of the optical signal from the transmitter to the receiver and to determine therefrom a position of the plunger within the cylinder portion of the drug container.