Using 4th- and 8th-grade mathematics data from 2003, 2007, and 2011 Third International Mathematics and Science Study (TIMSS) assessments, researchers conducted a cross-national empirical study to examine teacher participation in professional development and its impact on student achievement. Findings from the research show that, although 4th- and 8th-grade students in the United States had more access to teachers who participated in professional learning than similar students in other countries, one-third to one-half of 4th graders were taught by teachers who had no professional development in mathematics. Teachers' participation in professional development was positively associated with student achievement. Empirical research on the impact of professional learning on student achievement continues to be limited despite an increased emphasis on the importance of professional learning and federal guidelines defining levels of investment in professional learning for various federally funded initiatives. This study focuses specifically on six areas of professional learning in mathematics: math content, math pedagogy, math curriculum, integrating information technology into math, math assessment, and improving students' critical thinking or problem-solving skills.