Background: There is a continuing need to examine the relationship between diet quality and health in the population. The Healthy Eating Index (HEI) has been developed as a composite measure of diet quality by the US Department of Agriculture. Objectives: The first objective was to use the HEI to assess the diet quality of a representative sample of the US population and population groups. The second objective was to examine the association between HEI and obesity. Design: Cross-sectional analysis of data from 10 930 adults who participated in the Third National Health and Nutrition Examination Survey. Sociodemographic, physical activity, dietary, and health data were used in the analysis. Diet quality was assessed with the HEI score, ranging from 0 to 100, based on 10 dietary criteria. A low HEI score indicates poor diet. Results: A majority of survey participants had a low HEI score. The percentage of individuals classified as having a poor diet varied by age, gender, race/ethnicity, income, and education. A low HEI score was associated with overweight and obesity. There was a graded increase in the odds ratio of obesity across the HEI category after adjusting for age, gender, race/ethnicity, physical activity, smoking, alcohol use, income, and education. Conclusions: An index of diet quality, such as HEI, may provide a comprehensive assessment of diet in the population. Since the HEI is based on the US Dietary Guidelines, the use of these guidelines as a way to improve health should be emphasized. However, the overall effectiveness of these guidelines in disease prevention needs to be studied further. Sponsorship: None.