This research represents an investigation into user acceptance of computer systems. It starts with the premise that existing systems do not fully meet user requirements, and are therefore rejected as 'difficult to use'. Various problems and influences affecting user acceptance are identified, and improvements are suggested. Although a broad range of factors affecting user acceptance are discussed, emphasis is given to the impact of actual computer software. Initially, both general and specific user interface software influences are examined, and it is shown how these needs can be met using new software technology. A new Intelligent Interface architecture model is presented, and comparisons are made to existing interface design approaches. Secondly, the role of empirical work within the field of Human Computer Interaction is highlighted. An investigation into the usability and user. acceptance of a large working library database system is described, and the results discussed. The role of Systems Analysis and Design and its effect upon user acceptance is also explored. It is argued that despite improvements in interface technology and related software engineering techniques, a software application is also a product of the Systems Analysis and Design process. Traditional Systems Design approaches are examined, and suitable improvements suggested based upon experience with emerging separable software architectures. Thirdly, the research proceeds to examine the potential of Quantitative User Modelling, and describes the implementation of an example object oriented Quantitative User Model. This is then evaluated in order to determine new knowledge, concerning the major issues surrounding the potential application of user modelling to interface design. Finally, attention is given to the concept of interface and application separation. An object oriented User Interface Management System is presented, and its implementation in the Smalltalk 80 programming language discussed. The proposed User Interface Management System utilises a new software architecture which provides explicit user interface separation, using the concept of a Pluggable View Controller. It also incorporates an integrated design Tool-set for Direct Manipulation interfaces. The proposed User Interface Management System and software architecture represents the major contribution of this project to the growing body of Human Computer Interaction research. In particular, the importance of explicit interface separation is established, and the proposed software architecture is critically evaluated to determine new knowledge concerning the requirements, constraints, and potential of proper user interface separation. The implementation of an object oriented Part Hierarchy mechanism is also presented. This mechanism is related to the proposed User Interface Management System, and is critically evaluated in order to add to the body of knowledge concerning object oriented systems.