This article advances a continuing line of inquiry into an innovative teacher-support program intended to help in-service history teachers develop professional teaching knowledge for inquiry-based history instruction. Two prior iterations informed our design and use of professional development materials; they also informed the implementation schedule of experiences our materials intended to support. We asked: Can interactive and collaborative professional development encounters promote in-service history teachers' professional teaching knowledge? Following the thirteen-month study, no participant fully adopted the wise-practice pedagogy we advocated: problem-based historical inquiry. Findings do suggest, however, interactive experiences and sustained collaboration may help teachers raise their aspirational goals for planning powerful instruction. Throughout the study, teachers' collaboratively planned lessons became increasingly informed by our support program, and several participants' practice moved closer to our wise-practice model. Findings also suggest revisions to professional development materials and schedules to promote teachers' professional teaching knowledge.