The United States government has invested nearly one billion dollars in funding to professional historians and history educators across the country since 2000 to strengthen the teaching of American history in elementary and secondary schools, yet we know little about how these programs impact student learning. Using data from one such Teaching American History (TAH) grant, the authors employ multilevel models to investigate the effects of professional development on students' written responses to document-based questions at the fifth, eighth, and eleventh grades, and qualitative analyses of teachers' activities to learn about connections between classroom lessons and student outcomes. Findings indicate that fifth and eleventh grade students whose teachers were involved in ongoing networking activities for at least 30 hours in one year resulted in improved student performance. In addition, during the year, teachers of successful students engaged in activities that allowed them to increase their content knowledge and broaden their approach to teaching with primary documents. (Contains 6 tables and 5 notes.)