This paper describes an approach to cognitive engineering which promotes a symbiosis between the theories and methodologies of cognitive psychology and the practices of human-computer interaction design. We ground the description of our approach in a particular design problem: the development of computerised decision support in medical intensive care. We review the psychological literature of medical reasoning and decision making, highlighting its potential to inform the design of medical computerised aids. We also discuss how addressing this design problem may in turn benefit cognitive theory. This is followed by a brief description of our proposed methodology.