The main argument· of this chapter is that the understanding and assessment of a theory about the development of intellectual competence is dependent on its interpretation. Piaget's theory is a case in point. Developmentalists are not always mindful of the fact that there are several, competing interepretations of this theory. According to one (age-of-acquisition) interpretation, Piaget's theory concerns the ages at which intellectual competence is acquired. According to a second (differentiation-over-time) interpretation, Piaget's theory concerns how new forms of intellectual competence emerge over time. Developmentalists who accept the first interpretation often take Piaget's theory to be misleading or even incorrect. Developmentalists who accept the second interpretation often take questions about the validity and utility of Piaget' s theory to be open. Elaboration of this difference with special attention to its implications for resolving currently interesting questions about appropriate criteria for competence will be the central task in this chapter.