As research on clinical nutrition has become more concerned with the effects of macroand micronutrients on cognitive and brain development, success in evaluating and interpreting those effects is critically dependent on how human cognitive development is conceptualized and measured. The body of research on neurocognitive development from the past 50 years indicates that various cognitive components are relatively independent of one another and develop at different times during infancy and early childhood. For many studies in this area, however, the choice of measures of cognitive development for inclusion in clinical trials has not been guided by a particular theory of cognition or on the hypothesized effect of the nutrient. This practice is potentially disadvantageous for the interpretation of studies in the field; studies may choose neurocognitive assessments which may either obscure the specific effects of a particular nutrient or miss such specific effects altogether because the appropriate domain was not assessed. In developmental studies, this complex scenario is further compounded by the consideration of age-appropriate assessments and domains. This chapter will describe the difficulties in choosing and interpreting cognitive assessments for this field and make recommendations for best practices in addressing this issue.