We present a categorical mechanism to model interfaces in concurrent modular object systems. The behaviour of objects is modelled by sequential labelled prime event structures. Objects can be grouped into modules denoting system parts which are modelled by labelled event structures. The behaviour of systems is achieved from its interconnected modules by combining compositional and hiding mechanisms. A forgetful functor from the category of labelled prime event structures into the category of labels is introduced and proved to be a cofibration.