Background: Children's early skills are essential for their later success in school. Recent evidence highlights the importance of early mathematics, relative to reading and socioemotional skills, for elementary school achievement. Key advocacy groups for both early childhood and mathematics education have issued position statements on the importance of early mathematics, arguing that mathematics education for 3- to 6- year olds is essential to promoting future mathematics achievement. Focus of Study: Despite the fact that advocates and researchers are focusing on early math skills, we are still learning about the mathematics knowledge and skills young children typically have and how these early skills affect later academic achievement and school success. This study aims to address these gaps in the extant research by investigating how early math skills predict later school success. We explore how early math skills relate to achievement, from kindergarten through eighth grade, across reading, math, and science test score outcomes, as well as grade retention and teacher-reported academic achievement. We also explore whether there is variation in the relationship between early math skills and later outcomes for children who enter school with limited math skills. Research Design: We conduct secondary analysis with data from the Early Childhood Longitudinal Study-Kindergarten Cohort, a longitudinal, nationally representative sample of children who were in kindergarten in 1998-1999 and were followed through eighth grade. Results: We find that early math skills predict reading, math, and science achievement as well as grade retention from kindergarten through eighth grade. Results show that kindergarten math skills in pattern recognition, measurement, and advanced number are most predictive of eighth-grade outcomes overall and for subgroups including students who enter school with low math skills. The importance of these math skills for subsequent achievement increases or is maintained over time. Conclusions: The results reported here have implications for education policy regarding mathematics instruction in the earliest years of schooling. The fact that early mathematics knowledge and skills are the most important predictors not only for later math achievement but also for achievement in other content areas and grade retention supports a greater emphasis on mathematics than is currently the case in many kindergarten classrooms. It also suggest the possibility that focusing more on advanced number, pattern recognition, and measurement might develop skills that will benefit students in the later years of schooling.