This report describes the plan and operation of the third National Health and Nutrition Examination Survey. The sample for this study of the U.S. population was selected from households in 81 counties across the United States. About 40,000 persons 2 months of age and over were selected, including large samples of both young and old persons. About 12,000 of the sample persons were black Americans, 12,000 were Mexican-Americans, and the remaining 16,000 were of all other race and ethnicity groups. All selected persons were asked to complete an extensive interview and an examination in a large mobile examination center. The survey period is 1988-94, consisting of two phases of equal length and sample size. Both Phase 1 and Phase 2 were random samples of the U.S. population living in households.