The objective of this study is to describe the components of nutrition assessment in the National Health and Nutrition Examination Survey (NHANES) 1999-2002. The study design was a cross-sectional survey with a nationally representative sample of the US population. The survey participants were interviewed and completed a physical examination. From 1999 to 2002, a total of 25,316 people were included in the eligible sample, 21,004 people (83%) were interviewed, and 19,759 people (78% of the eligible sample) were examined. Dietary assessment consisted of a 24-hour dietary recall interview and questions on supplement use, food security, food-program participation, and other behaviors. Nutrition assessment included anthropometric measurements and body-composition assessment. A number of nutrition biochemistries were measured in blood and urine specimens. In addition, an assessment of cardiovascular fitness and questions on physical activity were included. Data are used to estimate population reference distributions and to monitor trends over time. Data have been used to evaluate the adequacy of nutrient intake using the Dietary Reference Intakes, to assist in development of nutrition policies related to obesity, and to evaluate policies such as folic acid fortification. The NHANES contributes to the knowledge and understanding of nutrition and health status in the US population through public-use microdata files for use by researchers in academia, in the private sector, and in government agencies. Continuous data collection will allow the NHANES to provide more timely information for policy development and evaluation.