Lee Cronbach once expressed the view that all roads lead to construct validity. In looking to clarify the consensus definition of validity, and its place in assessment, Newton is also led to the troublesome idea of construct validity. To be sure, he addresses other validity issues, but in this commentary, I will restrict my attention to construct validity. I will suggest that efforts to clarify our understanding of a domain of interest by making definitions of key terms more precise is of limited value and that the essentially contested idea of construct validity should be thought of as centrally concerned with theory validation. This will lead to a brief consideration of some methods of theory validation.