This article shares principles for offering effective professional development for classroom teachers at historic sites and museums, identified through ongoing research at the Thomas Jefferson Foundation (TJF). Supported by an IMLS National Leadership Grant, the initial findings challenged the education staff to more fully embrace their roles as teacher educators and reorient their programming around teachers' unique professional needs. The value of rigorous evaluation to program development and the potential utility of the evaluation materials developed at TJF for the field are also discussed.