The present invention relates to a medicament delivery device comprising a housing, a power unit (24, 26, 28), said power unit comprising: an actuation element (52); which actuation element (52) is operably arranged to move inside said housing ( ) for expelling a dose of medicament; said medicament delivery device further comprising a monitoring unit (30); said monitoring unit (30) being detachably attached to said housing; at least one sensor (32, 54) arranged in said monitoring unit (30), said at least one sensor (32, 54) being operably arranged to monitor said power unit (24, 26, 28) for obtaining information regarding status of the medicament delivery device.