Why quantum theory?

This article investigates one of the fundamental issues confronting a field that investigates quantum interaction; namely why is it necessary? The need to investigate an interaction using a quantum formalism is argued to arise when the system under study is sufficiently complex. In particular, if the system is displaying contextual behaviour then a quantum approach often incorporates this behaviour very naturally. Thus, a way in which much of the disparte work in the field of quantum interaction can be both justified to the broader community and eventually unified is presented. The nature of contextual behaviour and its relationship to nonlocality is explored