The aim of Human Computer Interaction or HCI is to both understand and improve the quality of the users' experience with the systems and technology they interact with. Recent HCI research requirements have stated a need for a unified predictive approach to system design that consolidates system engineering, cognitive modelling, and design principles into a single 'total system approach.' At present, few methods seek to integrate all three of these aspects into a single method and of those that do many are extensions to existing engineering techniques. This thesis, however proposes a new behaviour based approach designed to identify usability problems early in the design process before testing the system with actual users. In order to address the research requirements, this model uses a new design notation called Genetic Software Engineering (GSE) in conjunction with aspects of a cognitive modelling technique called NGOMSL (Natural GOMS Language) as the basis for this approach. GSE's behaviour tree notation, and NGOMSL's goal orientated format are integrated using a set of simple conversion rules defined in this study. Several well established design principles, believed to contribute to the eventual usability of a product, are then modelled in GSE. This thesis addresses the design of simple interfaces and the design of complex ubiquitous technology. The new GSE approach is used to model and predict usability problems in an extensive range of tasks from programming a VCR to making a video recording on a modern mobile phone. The validity of these findings is tested against actual user tests on the same tasks and devices to demonstrate the effectiveness of the GSE approach. Ultimately, the aim of the study is to demonstrate the effectiveness of the new cognitive and engineering based approach at predicting usability problems based on tangible representations of established design principles. This both fulfils the MCI research requirements for a 'total system approach' and establishes a new and novel approach to user interface and system design.