Newly developed assessments of nutrition and exercise knowledge, attitudes, and behavior were administered to 383 eighth-graders. Evidence for the validity of assessment scores was evaluated with five findings. First, parent- and self-reported behaviors were similar and congruent for healthy eating and exercising but not for sedentary behaviors or unhealthy eating. Second, the theory of planned behavior structural model was confirmed for exercise and nutrition attitudes: Attitudes predicted exercising (R[superscript 2] = 0.34), healthy eating (R[superscript 2] = 0.21), sedentary behaviors (R[superscript 2] = 0.11), and unhealthy eating (R[superscript 2] = 0.09). Third, gender differences replicated previous research: Girls have better diets and nutrition attitudes whereas boys exercise more frequently. The assessments distinguished normal-weight from obese individuals and showed incremental validity in predicting school grades. The paper concludes with a discussion of the assessments' potential for informing interventions. (Contains 3 tables and 7 figures.)