The lack of demonstrable success of computer based decision support technologies in health care requires a re-examination of the assumptions that support initiatives into these technologies. There are many ways in which one could explain away this lack of success, for example lack of technological maturity, lack of appropriate computer record infrastructures, or indeed professional resistance to novel technologies. However, one can also look at the fundamental assumptions that are taken as given when designing decision support systems. These assumptions about the problems that decision support systems should assist with, and the way that assistance should be provided are open to question. Much of the current work in decision support system research and development is based upon outdated views of the clinical workplace, and it is likely that the failure of the technology is in large part due to this. What is required is a principled re-examination of clinical practice, aimed at identifying the ways in which clinical workers should really be assisted. Until this is done, decision support technology will continue to be mismatched to the needs of health care workers.