Clinicians make judgments under conditions of uncertainty. Decisionresearch has shown thatin uncertain situations individuals do not always act rationally, coherently, or to maximize theirexpected utility. Advocates of clinical guidelines believe that these guidelineswill eliminate some of thecognitive biases that the practitioner may introduce into the medical decision-making process inan attempt to reduce its uncertainty. Other physicians have grave doubts about guidelines' application in practice. Guideline implementation lags well behind their development. Studiesof practicing physicians and a survey of clinicians in one specialty and setting indicate thatexperienced clinicians may be implementing guidelines selectively. Many clinicians areconcerned that guidelines are based on randomized trials and do not reflect the complexity of thereal world, in which a decision's context and framework are important. Their reluctance alsomay be due to the difficulty of applying general guidelines to specific clinical situations. Theproblem will only increase in the future. The patients of the 21st century will be older and havemore complex disease states. Physicians will have more patient-specific therapies and need toexercise more sophisticated clinical judgment. They may be more willing to use guidelines inmaking those judgments if research can demonstrate guidelines' effectiveness in improvingdecision making for individual patients.