Determining at what points in development children and other developing organisms acquire particular competencies would appear to be the least of the tasks with which developmental psychologists might be charged (Chandler & Chapman , this volume) . In fact, controversies regarding the "true ages" at which various competencies develop have been among the most intractable of disputes in the history of research in child development . Such disputes typically have been debated in terms of measurement issues, beginning with the claim that refined methods enable the investigator to detect the competence in question at an earlier point in development. However, decades of methodological refinements have not succeeded in resolving the issues. Instead, the debate has become stuck on questions of measurement validity; whether successful performance on a given task in fact reflects the intended comeptence or something else entirely. The fact that investigators cannot agree on what is actually measured with different procedures is perhaps an indication that they differ , not merely on questions of appropriate methodology, but in their very conceptualizations of the competencies in question . In this chapter, this argument is pursued in the context of disputes regarding the validity of verbal explanations as criteria for cognitive competence .