The purpose of this paper is two-fold. First, the author wants to describe generally how the Teaching American History (TAH) grant stakeholders in the Plymouth Canton Schools prioritized a professional development model that emphasized historical thinking. Rather than describing specific institutes, programs, and curriculum projects, the author outlines the major principles that guided this effort. Second, he illustrates how a disciplinary tool is constructed, through working with teachers, to scaffold professional development around historical thinking skills. Called the Historical Inquiry Protocol (HIP), the tool was designed to help both teachers and students interrogate primary sources as evidence. The professional development principles described in this paper were generated through discussion within a small group of grant stakeholders, a "foundations council," assembled to discuss the overall purposes and progress of the grant. The work in Plymouth Canton Schools in creating a "culture of evidence" was far from perfect. Many teachers remained skeptical--viewing the TAH grant as yet another in a long line of teaching reforms. Others embraced the TAH grant mainly as an opportunity to learn more historical content or explore topics that already interested them. However, at least one quarter of the teachers have become more curious and informed about the epistemology of history. These teachers have become more interested in how to frame the past as a problem, how to incorporate primary sources as evidence, and how to use the HIP as a prompter and question generator for the effective interrogation of these sources. Historical Inquiry Protocol (HIP) is appended. (Contains 3 notes.)