The series of papers presented in this issue about Mathematical Knowledge for Teaching (MKT) measures provide a clear and understandable presentation of the issues involved in the validation of new instruments. The authors' commitment to continuing validation studies leads everyone to believe their efforts will result in a high quality instrument. In this commentary, Lawrenz and Toal address three issues related to the validity argument of these authors. First, they argue that the purpose of the instrument needs to be more paramount in the validity argument. Second, they explain their concern regarding the separation of validation and theory development. Finally, they expand on the authors' call for more rigorous, financially supported validation studies in education.