A drug delivery system comprising a drug delivery device and a sensor device configured to be removably attached to the drug delivery device, the drug delivery device comprising: a first movable element which is configured to move along a path parallel to a longitudinal axis of the drug delivery device and which supports thereon a conductive region; and a rotatable element comprising encoded information that is externally visible on the rotatable element, the sensor device comprising: an array of conducting elements each forming part of a respective resonant circuit and each operable to have a signal applied to it, wherein the array of conducting elements are arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, each resonant circuit is operable to output a signal indicative of the proximity of the conductive region supported on the first moveable element to the respective conducting element of each resonant circuit; circuitry configured to receive the signals output from the resonant circuit and, based on the received signals, to determine information associated with a location along the path of the first movable element; and a sensing arrangement arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, the sensing arrangement is operable to read the encoded information that is externally visible on the rotatable element, wherein the circuitry is further configured to determine, based on the encoded information, a rotational orientation of the rotatable element corresponding to a drug dose to which the drug delivery device is currently dialled. the drug delivery device comprising: a first movable element which is configured to move along a path parallel to a longitudinal axis of the drug delivery device and which supports thereon a conductive region; and a rotatable element comprising encoded information that is externally visible on the rotatable element, a first movable element which is configured to move along a path parallel to a longitudinal axis of the drug delivery device and which supports thereon a conductive region; and a rotatable element comprising encoded information that is externally visible on the rotatable element, the sensor device comprising: an array of conducting elements each forming part of a respective resonant circuit and each operable to have a signal applied to it, wherein the array of conducting elements are arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, each resonant circuit is operable to output a signal indicative of the proximity of the conductive region supported on the first moveable element to the respective conducting element of each resonant circuit; circuitry configured to receive the signals output from the resonant circuit and, based on the received signals, to determine information associated with a location along the path of the first movable element; and a sensing arrangement arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, the sensing arrangement is operable to read the encoded information that is externally visible on the rotatable element, wherein the circuitry is further configured to determine, based on the encoded information, a rotational orientation of the rotatable element corresponding to a drug dose to which the drug delivery device is currently dialled. an array of conducting elements each forming part of a respective resonant circuit and each operable to have a signal applied to it, wherein the array of conducting elements are arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, each resonant circuit is operable to output a signal indicative of the proximity of the conductive region supported on the first moveable element to the respective conducting element of each resonant circuit; circuitry configured to receive the signals output from the resonant circuit and, based on the received signals, to determine information associated with a location along the path of the first movable element; and a sensing arrangement arranged within the sensor device such that, when the sensor device is attached to the drug delivery device, the sensing arrangement is operable to read the encoded information that is externally visible on the rotatable element, wherein the circuitry is further configured to determine, based on the encoded information, a rotational orientation of the rotatable element corresponding to a drug dose to which the drug delivery device is currently dialled.