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