A logging device (100, 300, 400) adapted to be releasably attached to a drug delivery device, the drug delivery device comprising a drug reservoir or means for receiving a drug reservoir, drug expelling means comprising dose setting means allowing a user to set a dose amount of drug to be expelled, and a generally cylindrical coupling portion, the logging device comprising: - electronic circuitry adapted to create a log of expelled dose amounts of drug, comprising: - sensor means adapted to capture, when the logging device is attached to the drug delivery device, a property value related to the dose amount of drug expelled from a reservoir by the expelling means during an expelling event, wherein the logging device comprises at least one individual snap lock adapted to engage the drug delivery device, the individual snap lock comprising a flexible wire portion (461, 561) adapted to be moved laterally and subsequently snap inwardly, whereby a snap lock may be provided between the logging device and the drug delivery device, characterized in that the generally cylindrical coupling portion has at least one lateral projection, and the logging device comprises a generally cylindrical bore adapted to receive the coupling portion, wherein: - the at least one individual snap lock is adapted to engage the lateral projection on the drug delivery device, - the flexible wire portion (461, 561) is arranged circumferentially in the bore and being adapted to be moved laterally by the lateral projection and subsequently snap inwardly when the coupling portion is inserted axially in the bore, and - at least one individual snap lock further comprises a pair of circumferentially arranged support structures (516, 517) serving as fulcrum points for the wire portion when the wire portion is moved laterally by the drug delivery device lateral projection when the coupling portion is inserted axially in the bore, whereby an axial snap lock can be provided between the logging device and the drug delivery device. - electronic circuitry adapted to create a log of expelled dose amounts of drug, comprising: - sensor means adapted to capture, when the logging device is attached to the drug delivery device, a property value related to the dose amount of drug expelled from a reservoir by the expelling means during an expelling event, - sensor means adapted to capture, when the logging device is attached to the drug delivery device, a property value related to the dose amount of drug expelled from a reservoir by the expelling means during an expelling event, wherein the logging device comprises at least one individual snap lock adapted to engage the drug delivery device, the individual snap lock comprising a flexible wire portion (461, 561) adapted to be moved laterally and subsequently snap inwardly, whereby a snap lock may be provided between the logging device and the drug delivery device, characterized in that the generally cylindrical coupling portion has at least one lateral projection, and the logging device comprises a generally cylindrical bore adapted to receive the coupling portion, wherein: - the at least one individual snap lock is adapted to engage the lateral projection on the drug delivery device, - the flexible wire portion (461, 561) is arranged circumferentially in the bore and being adapted to be moved laterally by the lateral projection and subsequently snap inwardly when the coupling portion is inserted axially in the bore, and - at least one individual snap lock further comprises a pair of circumferentially arranged support structures (516, 517) serving as fulcrum points for the wire portion when the wire portion is moved laterally by the drug delivery device lateral projection when the coupling portion is inserted axially in the bore, - the at least one individual snap lock is adapted to engage the lateral projection on the drug delivery device, - the flexible wire portion (461, 561) is arranged circumferentially in the bore and being adapted to be moved laterally by the lateral projection and subsequently snap inwardly when the coupling portion is inserted axially in the bore, and - at least one individual snap lock further comprises a pair of circumferentially arranged support structures (516, 517) serving as fulcrum points for the wire portion when the wire portion is moved laterally by the drug delivery device lateral projection when the coupling portion is inserted axially in the bore, whereby an axial snap lock can be provided between the logging device and the drug delivery device.