This thesis is concerned with methods for the effective design and analysis of the content of Virtual Environments (YEs) and documents and collates a range of research from within the field of Virtual Reality (VR) as well as earlier work in the field of methods integration for software engineering. First of all the nature of VR and certain problems specific to the design of YEs are discussed. The hypothesis of the thesis is then stated: to the effect that there is a need for a practical method for analysing, comparing and designing YEs across a range of delivery platforms, application domains and genres. In order to realise the demands of the hypothesis a content model for YEs is proposed - Perceptual Opportunities (POs) - and later expounded after a set of criteria to guide it have been developed. Having established the PO model it is then applied to the analysis of extant YEs and then extended to their comparative content analysis: this by way of verifying the practical utility of the model. Following this the ongoing experimental verification of the model in collaboration with psychologists is discussed. The discussions then move on to focus on the wider issues of VE design and design methodology and the role of POs in this context. At each stage of the exposition the POs are assessed in terms of the content criteria and the demands of the hypothesis. The conclusions focus on a number of current and potential applications and extensions of the PO theory and finish with a call for the establishment of a discipline of 'VR Theory' for which POs are intended as a stimulating factor