The goals of this study were to examine the longitudinal relations between school readiness and reading and math achievement and to test if these relations were moderated by temperament. The sample included socio-economically and ethnically diverse twins (N=551). Parents reported on school readiness when children were five years old. Teachers reported on temperament (effortful control, anger, and shyness) three years later. Standardized measures of reading and math were obtained when children were eight years old. Effortful control and shyness moderated the effect of school readiness on reading. Prediction of reading from school readiness was strongest when students were high in effortful control and low in shyness. Effortful control and shyness predicted math beyond school readiness. There were no relations involving anger. Findings demonstrate that temperament can potentiate the relations between school readiness and reading and highlight the importance of promoting school readiness and effortful control, while decreasing shyness.