A sensor device is removably attachable to a drug delivery device. The sensor device comprises an array of optical sensors arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, which has a first movable element configured to move along a path parallel to the longitudinal axis of the drug delivery device, each optical sensor is operable to detect light received at different locations along the path and to output a signal indicative of an amount of detected light. Circuitry is configured to receive the signals output from the optical sensors and, based on the received signals, determine information associated with a location along the path of the first movable element.