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