A sensor device removably attachable to a drug delivery device includes a sensing arrangement arranged within the sensor device and circuitry configured to process signals provided by the sensing arrangement. The sensing arrangement is configured such that, when the sensor device is attached to the drug delivery device, the sensing arrangement is operable to read encoded information present on a rotatable component internal to the drug delivery device and that is able to be sensed through a window or aperture of the drug delivery device, and is operable to sense through the window or aperture at least a part of a drive mechanism of the drug delivery device. The circuitry is configured to determine information relating to a set dose, and to determine information relating to whether the drug delivery device is in a dose setting mode or whether the device is in a dose delivery mode.