A drug delivery device 100 comprising: a housing 102； a cylindrical member 406 rotatably supported within the housing； and a plurality of sensors 212a-212g； wherein: the outer surface of the cylindrical member is provided with a single track 300, the track forming an encoder and having a plurality of first track segments 302 and a plurality of second track segments 304 arranged along the length of the track which are respectively capable of inducing first and second responses in the sensors； and in each rotational position of the cylindrical member relative to the housing at least one different first track segment is capable of inducing a first response in at least one said sensor, thereby enabling the rotational position of the cylindrical member relative to the housing to be determined.
