Research Findings: Recent research and teacher reports have highlighted the importance of early behavior skills for children's school readiness and academic success in elementary school. Significant gaps in school readiness and achievement exist between children in poverty and those more affluent. Low-income children are also more likely to exhibit behavior concerns than their more financially advantaged peers. The current study examined the importance of behavior skills at age 4 for school readiness and academic achievement in kindergarten among an ethnically diverse sample of 1,618 low-income children (63% Latino, 37% Black) in an urban setting. Children's early behavior concerns at age 4 were significantly associated with children's school readiness scores and end-of-year kindergarten grades above and beyond the contributions of family and child demographics and children's early cognitive and language skills. In addition, behavior problems were more strongly related to school readiness and kindergarten performance within English-dominant Latino children as opposed to Spanish-dominant Latino children. Practice or Policy: The findings from the current study provide support for targeting behavior skills, and not just preliteracy and/or number skills, prior to school entry as a strategy to increase the likelihood of low-income diverse children's school readiness and school success. Behavior interventions are discussed.