Background: Prior research shows that teachers' working conditions are important for teacher and student outcomes, such as teacher retention and student achievement. Because National Board Certified Teachers (NBCTs) can be effective in the classroom and as instructional leaders, they are well positioned to create favorable working conditions for all teachers. Therefore, having NBCTs at a school could strengthen working conditions in ways that result in improved teacher and student outcomes. For positive outcomes to be realized, however, the concentration of NBCTs at a school may need to increase. No empirical study has directly examined the relationship between the concentration of NBCTs at the school level, and teacher and student outcomes--a gap in knowledge that the current study begins to address. Purpose: The study examines the relationship between the concentration of NBCTs at a school, operationalized as the proportion of NBCTs in teaching roles relative to all teachers, and student achievement in mathematics and English language arts in Grades 4-8 and teacher retention in Grades K-8. The outcome of teacher retention is for non-NBCTs. Therefore, it represents a spillover effect of NBCTs. Setting: The study was conducted in North Carolina and Kentucky. Research Design: The study is correlational. Findings: The evidence of the relationship between the concentration of NBCTs at a school and student achievement is not compelling: We found some statistically significant relationships in both states, but the estimates are inconsistent and small in statistical and practical terms. The direct relationship between the concentration of NBCTs and retention of non-NBCTs is not significant in either state. However, the concentration of NBCTs is positively and significantly associated with the retention of non-NBCTs at schools serving a high proportion of economically disadvantaged students in North Carolina compared with schools serving a low proportion of economically disadvantaged students. The estimate of this relationship is the strongest in the current study. The relationship in Kentucky is not significant. Conclusions/Recommendations: The pattern of results for teacher retention in North Carolina in the current study is encouraging. It suggests that increasing the concentration of NBCTs may be one possible avenue for keeping teachers teaching at the same schools, including schools serving a large proportion of economically disadvantaged students where teacher turnover tends to be high, negatively affecting teachers and students. Overall, we conclude that the study's findings are sufficiently compelling to warrant additional research to examine NBCT concentration using multiple years of data and with more rigor than the current study could do.