A sensor device to be attached to a drug delivery device includes an array of conducting elements each forming part of a respective resonant circuit and each operable to have a signal applied to it. The drug delivery device has a first movable element supporting a conductive region, which is configured to move along a path parallel to a longitudinal axis of the drug delivery device. When the sensor device is attached to the drug delivery device, the conducting elements are arranged such that 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. The sensor device includes circuitry to receive the signals output from the resonant circuit and, based on the received signals, determines information associated with a location along the path of the first movable element.