Some cancer screening and treatment decisions are not clear cut because outcomes are uncertain or options have different benefit/risk profiles. "Decision aids" have been developed as adjuncts to counseling so that patients can learn about benefits and risks, can consider their personal values, and can participate with their practitioner in decision making. The purpose of this paper is to review published evidence about the efficacy of decision aids focused on cancer outcomes and to outline research and dissemination issues. Studies evaluating cancer-related decision aids demonstrate that they are acceptable to patients and help those who are uncertain at baseline to make choices. They also increase the likelihood that choices are based on better knowledge, realistic expectations of outcomes, and personal values. Decision aids reduce some dimensions of decisional conflict, and their effect on decisions is variable. Few studies examine the downstream effects of decision aids on long-term persistence with choices, regret, and quality of life. The differences between simpler and more intensive methods of decision support appear to be negligible in terms of knowledge and satisfaction as well as variable in terms of decisions and decisional conflict. However, more intensive methods are superior in terms of user acceptability and of the extent to which choices are based on realistic expectations and personal values. The clinical importance of these differences and the cost-effectiveness remain to be established. On the basis of this review, several recommendations for research are made, and dissemination issues are identified.