The intellectual contribution of this thesis lies within the area of computer supported co-operative work (CSCW), and more specifically, computer supported co-operative design (CSCD). CSCW is concerned with the development of information systems and technological support for multi-participant work activities. Research into CSCW seeks to understand how people and organisations interact with one another, and to integrate this understanding with the development of computer based tools to support real world settings. Much of the technology developed to support the work of designers has been developed to aid individuals working alone, with tools like computer aided drafting (CAD), scheduling, and database software. The growth of interest in ‘groupware’ has led many technology developers to adapt these design tools for use in group situations. However, joint activities are different from those performed alone, and organisational structures can both interfere with, and supplement co-operative work practices in a way that the current technologies cannot provide support for. To develop effective group design tools, we need to understand more about collaborative processes in design. This thesis draws from the theoretical underpinning of cognitive science and the methods of anthropology and sociology, in an interdisciplinary study of design performance in the construction industry. Fieldwork is used as a method of qualitative data collection and this is examined within the analytic framework of distributed cognition. The results of this analysis provide a useful and usable description of the work of design that technology developers can use to support collaborative design work. In line with the methods of distributed cognition, the activities observed in the workplace studies are examined in terms of their processes and representations. The resources that were available to the design participants are made explicit, as are their situation-specific work patterns. Two case studies of design are examined. The first of these describes design work in a civil engineering project, which involves a number of different design activities. The second describes the work of consulting engineers in building design, focusing on a more limited design role, which is used to back up and supplement areas of the first study that were understood to be particularly relevant. The findings of the study demonstrate how design processes operate simultaneously at personal, organisational and inter-organisational levels. The distinction between the formal, organisational procedures, and the informal, social processes that compliment them is examined to show how these are interrelated in the performance of the design task and their importance to the mechanisms used to co-ordinate actions. The findings of the study have implications for the development of novel technologies to augment the engineering design process, and have already been used in the development of assistive design technologies. The thesis demonstrates that the framework of distributed cognition can be used in the analysis of cognition within a setting, involving multiple individuals, in concert with 'natural' and 'artificial' artefacts. The thesis makes clear a number of processes in design that can only be examined from a perspective which includes the social dimensions of work. The methods of study focus on the resources in collaborative activities, whilst the analysis, structured in terms of the representations and processes of collaborative activity, shows that the method can be used effectively in the development of CSCW and CSCD technologies.