An assembly comprising: (a) a drug delivery device having or being adapted to receive a reservoir for a drug, comprising: - an outlet for the drug, - a drug expelling mechanism for expelling drug from the reservoir and out through the outlet, - a housing in which at least a portion of the drug expelling mechanism is arranged, and - first coupling means for a cap, (b) a cap adapted to cover the outlet, comprising: - second coupling means for engaging the first coupling means, and (c) a monitoring device comprising: - a housing portion, - means for detecting and registering an action performed in the drug delivery device, and - third coupling means adapted to engage the drug delivery device, - whereby the monitoring device can be arranged in a pre-determined mounted position relative to the drug delivery device when the third coupling means engages the drug delivery device, characterized in that - the third coupling means is adapted to engage the first coupling means on the drug delivery device, whereby the monitoring device can be arranged in the pre-determined mounted position relative to the drug delivery device when the first and third coupling means engage each other, this allowing information to be transferred between the drug delivery device and the monitoring device, - the monitoring device does not cover the outlet when arranged in the mounted position, thus allowing an amount of drug to be expelled from the reservoir while the monitoring device is coupled to the drug delivery device, and - the means for detecting is adapted to detect the size of a dose being expelled. (a) a drug delivery device having or being adapted to receive a reservoir for a drug, comprising: - an outlet for the drug, - a drug expelling mechanism for expelling drug from the reservoir and out through the outlet, - a housing in which at least a portion of the drug expelling mechanism is arranged, and - first coupling means for a cap, - an outlet for the drug, - a drug expelling mechanism for expelling drug from the reservoir and out through the outlet, - a housing in which at least a portion of the drug expelling mechanism is arranged, and - first coupling means for a cap, (b) a cap adapted to cover the outlet, comprising: - second coupling means for engaging the first coupling means, and - second coupling means for engaging the first coupling means, and (c) a monitoring device comprising: - a housing portion, - means for detecting and registering an action performed in the drug delivery device, and - third coupling means adapted to engage the drug delivery device, - a housing portion, - means for detecting and registering an action performed in the drug delivery device, and - third coupling means adapted to engage the drug delivery device, - whereby the monitoring device can be arranged in a pre-determined mounted position relative to the drug delivery device when the third coupling means engages the drug delivery device, characterized in that - the third coupling means is adapted to engage the first coupling means on the drug delivery device, whereby the monitoring device can be arranged in the pre-determined mounted position relative to the drug delivery device when the first and third coupling means engage each other, this allowing information to be transferred between the drug delivery device and the monitoring device, - the monitoring device does not cover the outlet when arranged in the mounted position, thus allowing an amount of drug to be expelled from the reservoir while the monitoring device is coupled to the drug delivery device, and - the means for detecting is adapted to detect the size of a dose being expelled.