Enhancing teachers' ability to assess and articulate claims of historical significance will provide a valuable compass that thoughtful teachers use to navigate large amounts of material in meaningful ways. This study explores how and to what extent--if any--teachers develop their ability to apply historical reasoning to determine the significance of historical people, places, events, or ideas after working closely with historians during a professional development seminar designed to increase their knowledge and skills as American History teachers. Participating teachers attended a ten-day summer institute, during which historians presented current historical scholarship about nineteenth-century America with a particular focus on the complexities of racial relationships before and after the Civil War. In this study, the authors investigate changes in teachers' abilities to use historical thinking skills to make and support claims of historical significance by analyzing a document-based assessment given before and after the professional development course. The findings suggest that teachers can develop the skills associated with making claims about the historical significance of a given primary source. It is worth noting that teachers demonstrated improvement in this task following intensive professional development which was intended to increase their content knowledge and historical thinking skills in general. (Contains 1 figure and 31 notes.)