The research involves the testing of Overall and Experimental Hypotheses concerning the nature of design activity and the relationship between designers' cognition and their use of CAD systems. In particular, a lack of understanding of the design process was perceived together with a need to utilize aspects of designers internal problem solving capabilities in the specification of electronio design aids. A multidisciplinary approach is used in review and experimental methods in order to associate theoretical aspects of design behaviour with the measurement of practice. The social, economic and industrial implications of new technology are reviewed in the light of their e~~ects upon intellectual industrial tasks, such as designing within the textile and other industries. Also considered are the theoretical aspects o~ designing and in particular the designers' cognitive activity is described in behavioural terms. Such theories are tested in experimental situations where the problem solving aspects are elicited from designers in relation to their internal schematic representation o~ design problems. Speci~ic conclusions in the area o~ person-system interaction concern both Hypotheses. Adaptation was ~ound to be best considered as another aspect of the holistio representation that a designer has of his task. This resulted ~rom a recognition o~ the individuals own methods o~ problem solving within the con~ines of the task. The use o~ repertory grid techniques ~or the investigation of designing did not prove however that a designer's internal spaoe is completely measurable. Indeed, it resulted that some phenomena associated with design appear ~undamentally subjective although it was £ound that identi~iable and individual oriteria are used by designers. The particular contribution that this work makes is in the assooiation o~ psychological, sociologioal and industrial aspects o~ designing in an approach to the adaptive behaviour of designers in their use of CAD "systems