Although mathematical skills are important to economic success in this society, U. S. students routinely perform below international standards of math achievement. Given such findings, there is a pressing need to understand factors that contribute to individual and group weakness in mathematics before such difficulties become entrenched. By studying preschool children with a longitudinal approach, the current study aimed to improve understanding of math development by investigating the unfolding relationship between math interest and achievement. Based on research with older children, it was expected that math interest and skill would be both concurrently related and predictive of one another over time. Additionally, research from the self-efficacy literature suggests that a child's conception of his or her math ability would be related to both the child's math interest and actual skill. Using the TEMA-2 as an assessment of skill and a multimodal approach to measuring interest, this study explored the measurement of math interest in young children. Gender and ethnic differences were found in select teacher measurements of interest, though none were found on observed or child-reported interest. Concurrent relationships were found between the different measures of interest and math ability. Even when controlling for initial skill or interest, skill was predictive of later observational ratings of math interest, and both observational and teacher measurements of interest were predictive of later skill. Because the assessment of self-efficacy demonstrated poor psychometric properties, further analyses were not conducted. (author abstract)