In order to improve students' opportunities to learn, educators need tools that can assist them to reflect on and analyze their own and others' teaching practice. Many available observation tools and protocols for studying student work are inadequate because they do not directly engage educators in core issues about rigorous content and pedagogy. In this conceptual paper, we argue that the Instructional Quality Assessment (IQA)--a formal toolkit for rating instructional quality that is based primarily on classroom observations and student assignments--has strong potential to support professional development within schools at multiple levels. We argue that the IQA could be useful to "teachers" for analyzing their own and their colleagues' practice; additionally, the IQA could aid the efforts of "principals" in their work as instructional leaders, identifying effective practitioners to help lead professional development within a school and targeting professional development needs that would require external support. Although the IQA was designed for summative, external evaluation, we argue that the steps taken to improve the reliability of the instrument--particularly the efforts to make the rubric descriptors for gradations of instructional quality as transparent as possible--also serve to make the tool a resource for professional growth among educators. The following are appended: (1) Abridged Version of the Principles of Learning; (2) Relationship between Checklist Ratings and Rubric Scores; and (3) Accountable Talk Function Checklist. (Contains 3 notes, 1 table, and 1 figure.)