This talk will report work on HCI modelling for virtual reality applications and at a meta level discuss the approach we are taking to developing theory in HCI and applying the same to design practice. First I will describe the VR interaction models which are a development of Norman's theory of action to account for the increased perceptual and motor possibilities in virtual environments. The interaction models form part of a theory of interaction which links knowledge sources and affordances supplied by an interface design (termed generic design properties) which are necessary for successful interaction in a given task context. The theory and its application as a walkthrough usability analysis method will be described with some examples taken from a VR simulation system. If time allows work in progress on theory validation via 'process experiments'- a protocol studies in semi controlled conditions, and implementation of the theory in a computational cognitive architecture, COGENT, (Cooper et al 1996) will be discussed.