The thesis focuses on the relations between design drawings and design activity. It develops an approach to conceptual processes employed during designing and their impact on drawing operations, and examines the implications of this approach on the systemic representation of drawings in computerised environments. The implementation of most computerised drawing systems relies on approaches that treat drawings as symbolic structures determined mainly by geometric knowledge. The thesis takes the view that drawing is an integral part of designing and it looks at: the role of drawn descriptions within conceptual activity aimed at design information; representational schemes developed for the formation and interpretation of graphical representations; and qualifications that such schemes exhibit during the accomplishment of design tasks. It argues that the ways in which design drawings are used reveal much of the knowledge used in designing, and it is this knowledge which determines the structure of drawings. A theme throughout the thesis is the discussion of a case study concerning the development of a design project by students of architecture using a computerised drawing system. Although emphasis is placed on drawing systems, the contribution of the thesis lies in its global understanding of the effects of design processes on drawings, and this understanding might be of interest to future advanced design systems incorporating graphical representations.