Systems and methods for monitoring an operational state and/or a fill status of a drug container of a drug delivery device are provided. The drug container can hold a liquid drug. A plunger can be positioned within the drug container. A drive system can advance the plunger to expel the liquid drug from the container. A monitoring system can detect a movement and/or a position of the plunger and/or any component coupled to the plunger. The detection can enable determination of an amount of liquid drug that has been expelled and/or an amount of liquid drug remaining in the drug container. Dosing rates, flow rates, and dosage completion can also be determined.