Medical informatics has experienced dramatic growth, both as an applied and as a research discipline in recent years. In this paper, we argue that there is a need to expand the research base to characterize the cognitive dimension of informatics. Theories and methods from cognitive science can provide an effective counterpart to traditional medical informatics in addressing issues of usability of the systems, the processing of information, and the training of physicians. In the first part of the paper, we address the problems inherent in applying basic theories to practice and suggest some potential solutions. The second section deals with epistemological issues that are fundamental to cognitive science research and medical informatics. We then discuss two areas of application of cognitive scientific theories and methods to medical informatics: cognitive evaluation of human computer interface and intelligent medical decision support systems. The paper addresses the progress that has been made thus far and discusses how future cognitive research can facilitate further growth in the development of these applications.