An assembly including a drug delivery device and a monitoring device adapted to detect an action taking place in the drug delivery device. The drug delivery device including a main portion having a drug expelling mechanism for expelling drug from the reservoir through an outlet, a housing in which at least a portion of the expelling mechanism is arranged, and a member able to attach a cover to the drug delivery device. The cover is adapted to cover the outlet when mounted. The monitoring device includes a housing portion, a member for detecting an action taking place in the drug delivery device, and at least one member adapted to engage a member present on the main portion. By this arrangement the monitoring device can be placed in a pre-determined position relative to the main portion, allowing efficient transmission of information between the monitoring device and the main portion.
