Teacher knowledge has been of theoretical and empirical interest for over two decades, and development of measures is overdue. The researchers represented in this volume have been breaking new ground by developing a measure of mathematical knowledge for teaching (MKT) without guiding precedents, and in the face of differing perspectives on teacher knowledge--its content, organization, and relationship to practice (e.g., Davis, 2004; Magnusson, Krajcik, & Borko, 1999; Schoenfeld, 2000; Shulman, 1986). In this commentary, the author focuses on the constructs the MKT measure is designed to measure. The author frames her comments as a series of questions, because she remains unclear about the constructs as well as specifications for the items. How do the authors conceptualize ... .: (1) the mathematics in each of the targeted domains?; (2) learning goals--what is important for elementary students to know and be able to do in these mathematical domains?; (3) how children learn--how they build knowledge and competence in the targeted domains?; (4) effective elementary mathematics teaching practice?; and (5) teacher knowledge, and the relationship between knowledge and practice? The work reported in this volume will make a greater contribution to mathematics education, writes Gearhart, if the work is more clearly situated in related theory and research on mathematics teaching and learning, including research on the knowledge teachers need to teach effectively. (Contains 1 footnote.)