The latest developments in human computer interfaces aim at greater ease of use, and the exploitation of human communication and interaction skills typical of non-computerised environments. This kind of interaction is continuous rather than purely discrete. Continuous interaction implies a tighter coupling between system and user, and raises complicated synchronisation issues where real-time requirements and intrinsic variation of human behaviour play an essential role. In this paper, we propose a human centred layered reference model to reduce the design complexity of systems exhibiting continuous interaction. In the context of the layered model, we discuss the role that formal modelling can play in the design of these systems.