A drug delivery device comprising: a housing; at least four sensors (212a to 212g); and a cylindrical member supported within the housing, the outer surface of said cylindrical member being provided with a helical track, the helical track comprising track segments of a first type and track segments of a second type, the first and second types of track segment being respectively capable of inducing first and second responses in the sensors; wherein: the helical track has a width dimension; the helical track includes across its width at least one track segment of the first type and at least one track segment of the second type at plural positions along the length of the helical track; the device is configured such that during a first phase of a drug delivery operation the helical track is moved axially, without rotation, relative to the sensors between a first position and a second position, and during a second phase of the drug delivery operation the track is moved helically relative to the sensors from the second position; and responses induced in the sensors by the track segments of the helical track are different in the first position compared to responses induced in the sensors by the helical track in the second position. a housing; at least four sensors (212a to 212g); and a cylindrical member supported within the housing, the outer surface of said cylindrical member being provided with a helical track, the helical track comprising track segments of a first type and track segments of a second type, the first and second types of track segment being respectively capable of inducing first and second responses in the sensors; wherein: the helical track has a width dimension; the helical track includes across its width at least one track segment of the first type and at least one track segment of the second type at plural positions along the length of the helical track; the device is configured such that during a first phase of a drug delivery operation the helical track is moved axially, without rotation, relative to the sensors between a first position and a second position, and during a second phase of the drug delivery operation the track is moved helically relative to the sensors from the second position; and responses induced in the sensors by the track segments of the helical track are different in the first position compared to responses induced in the sensors by the helical track in the second position. the helical track has a width dimension; the helical track includes across its width at least one track segment of the first type and at least one track segment of the second type at plural positions along the length of the helical track; the device is configured such that during a first phase of a drug delivery operation the helical track is moved axially, without rotation, relative to the sensors between a first position and a second position, and during a second phase of the drug delivery operation the track is moved helically relative to the sensors from the second position; and responses induced in the sensors by the track segments of the helical track are different in the first position compared to responses induced in the sensors by the helical track in the second position.