Diseases associated with dietary excess and imbalance rank among the leading causes of illness and death in the United States. Major diseases in which diet plays a role include coronary heart disease, some types of cancer, stroke, hypertension, obesity, osteoporosis, and non-insulin dependent diabetes mellitus. All are major causes of morbidity and mortality in this country. Although diet is associated with multiple health outcomes, the relationships between specific dietary elements and specific health outcomes have been studied extensively. The role of the primary care provider in either providing direct diet counseling or enlisting the help of other health professionals has been studied extensively, but controversy remains about the effectiveness of different strategies. In .Evaluating Primary Care Behavioral Counseling Interventions: An Evidence-based Approach, . Whitlock et al. described a detailed framework for primary care counseling. To address the question of the role of diet in chronic disease as well as dietary assessment and counseling in primary care, staff of the RTI . University of North Carolina Evidence-based Practice Center undertook this systematic evidence review (SER) on behalf of the US Preventive Services Task Force (USPSTF). It updates the chapter on dietary counseling from the second edition of the Guide to Clinical Preventive Services. In 1996, the USPSTF had recommended counseling adults and children over 2 years of age to limit intakes of fat and cholesterol, to maintain caloric balance in diets, and to emphasize foods containing fiber; the Task Force concluded then that the evidence was insufficient to recommend for or against counseling the general population to reduce dietary sodium. The Task Force also concluded that evidence was insufficient to show that nutritional counseling by physicians has any advantage over counseling by dietitians or community interventions. This SER enabled the USPSTF to reconsider the issues it previously addressed and to make recommendations concerning ways to promote healthy dietary practices in America. To guide the work more precisely, we identified 7 key questions: 1. What is the relationship between dietary patterns and health outcomes? 2. What are valid, feasible tools for assessment of dietary risk? 3. What are the adverse effects of dietary assessment? 4. What is the efficacy of primary care counseling and dietary behavior change interventions? 5. What are the adverse effects and associated costs of dietary behavior intervention? 6. Which of the following system influences facilitate or impede dietary intervention: features of the health care team? features of the practice setting? features of the health care system? 7. Can dietary supplements improve nutrition in patients identified as undernourished