The present study investigated the effects of "Primarily Math", an inservice elementary mathematics specialist program. "Primarily Math" sought to augment the mathematical knowledge for teaching of kindergarten through third-grade teachers using a longitudinal multiple cohort design. Two sets of analyses were conducted. The first examined impact on teachers' mathematical knowledge for teaching, attitudes toward learning mathematics, and beliefs about teaching and learning relative to a matched comparison group. "Primarily Math" teachers demonstrated greater knowledge for teaching Numbers and Operations and more positive attitudes toward learning mathematics, and more often endorsed student-centered beliefs about teaching and learning. The second set of analyses examined the extent to which students of three cohorts of "Primarily Math" teachers demonstrated more fall-spring growth in a measure of mathematics achievement relative to students of comparison-group teachers. There was a small but positive effect of participation in "Primarily Math" on student mathematics achievement.