A secondary analysis of the Early Childhood Longitudinal Study-Kindergarten Sample (N = 17,401) was conducted to determine the factors that are most strongly associated with math achievement during kindergarten, first grade, and third grade. Factors from the following three categories were considered: antecedent factors (e.g., family socio-economic status), opportunity factors (e.g., frequency of being exposed to mathematical content), and propensity factors (e.g., pre-existing mathematics skills). Structural equation modeling showed that math achievement was strongly predicted by a combination of specific propensity, opportunity, and antecedent factors. However, propensity factors were the most important determinants of achievement. The amount of variance accounted for by gender and ethnicity was substantially reduced when other factors in the antecedent, opportunity, and propensity categories were controlled. The implications of the findings for intervention are discussed. (Contains 5 figures and 3 tables.)