Computer interface design has the primary purpose of assisting information technology users in their professional activities. In order to accomplish this, users need to be able to work 'through the interface' to complete the tasks which achieve the goals associated with an activity. Although this is the conceptual province of psychology, very little use has been made of psychology in practical interface design. The attempts that have been made appear to have suffered from a lack of connection to real life problems which has been attributed to their foundations in the information processing structure of cognitive psychology. We elaborate an approach to interface design based on the Russian developed activity theory which provides a more complete analysis of human nature and which avoids the problems inherent in the view of humans as exclusively information processors. The relationship of this theory to human computer interaction is considered and its relationship to interface design with activity theory providing a paradigm for the description and understanding of the way humans interact with computers within the context of their environment. A practical example of how this research arose from a problem encountered in an application to a military history interface design will also be discussed.