A medication delivery device comprising: a housing (135, 150) including a container retainer; a medication container held within the container retainer, the medication container including a barrel (50, 250,350) and a plunger (52, 252, 352), the barrel made of a light transmitting material and extending in an axial direction between a forward outlet end and a rearward end, the plunger having at least one peripheral surface region in fluid tight engagement with an interior surface of a wall (261, 361) of the barrel to seal medication in the barrel forward of the plunger; an advancing mechanism within said housing operable to advance said plunger within said barrel to dispense medication from the medication container through said forward outlet end; the medication delivery device being characterised by: a light source (30, 30', 162, 230, 330, 440) mounted within said housing to emit light into the barrel wall so that the barrel wall serves as a waveguide to guide the light to travel therein in the axial direction; a light detector (35, 35', 35", 175, 235, 335, 410) mounted within the container retainer to detect reflected light that was emitted by the light source, traveled through the barrel wall serving as the waveguide, and then reflected off the at least one peripheral surface region; and a controller mounted within the housing in communication with said light detector to determine an axial position of the at least one peripheral surface region within the medication container based on data from said light detector of said detected reflected light. a housing (135, 150) including a container retainer; a medication container held within the container retainer, the medication container including a barrel (50, 250,350) and a plunger (52, 252, 352), the barrel made of a light transmitting material and extending in an axial direction between a forward outlet end and a rearward end, the plunger having at least one peripheral surface region in fluid tight engagement with an interior surface of a wall (261, 361) of the barrel to seal medication in the barrel forward of the plunger; an advancing mechanism within said housing operable to advance said plunger within said barrel to dispense medication from the medication container through said forward outlet end; the medication delivery device being characterised by: a light source (30, 30', 162, 230, 330, 440) mounted within said housing to emit light into the barrel wall so that the barrel wall serves as a waveguide to guide the light to travel therein in the axial direction; a light detector (35, 35', 35", 175, 235, 335, 410) mounted within the container retainer to detect reflected light that was emitted by the light source, traveled through the barrel wall serving as the waveguide, and then reflected off the at least one peripheral surface region; and a controller mounted within the housing in communication with said light detector to determine an axial position of the at least one peripheral surface region within the medication container based on data from said light detector of said detected reflected light. a light source (30, 30', 162, 230, 330, 440) mounted within said housing to emit light into the barrel wall so that the barrel wall serves as a waveguide to guide the light to travel therein in the axial direction; a light detector (35, 35', 35", 175, 235, 335, 410) mounted within the container retainer to detect reflected light that was emitted by the light source, traveled through the barrel wall serving as the waveguide, and then reflected off the at least one peripheral surface region; and a controller mounted within the housing in communication with said light detector to determine an axial position of the at least one peripheral surface region within the medication container based on data from said light detector of said detected reflected light.