"When compared with high-achieving countries around the world, the U.S. appears to be significantly behind in providing certain kinds of professional learning opportunities" (Wei, Darling-Hammond, Andree, Richardson, & Orphanos, 2009, p. 39). The present study was designed to investigate the relationship between sustained, targeted professional development in mathematics and student performance in the United States and in the high-achieving countries listed in the Third International Mathematics and Science Study. The research for this study involved teachers and students in a consortium of school districts in the state of Ohio, formed to meet the challenge of closing the achievement gap in mathematics and science for students in the United States. The findings indicate that professional development can be seen as a significant contributing factor in the learning cycle and that sustained, targeted professional development has a positive effect on student achievement.