Interest in U-shaped development increased in November and December 2000 when a vigorous discussion took place on U-shaped patterns and their potential explanations. This debate led to a special issue devoted to the topic. The articles raise three main questions: Why are U-shaped curves of interest? What do U-shaped curves tell about the processes that produce cognitive development? What does the resurgence of interest in U-shaped curves tell about where the field of cognitive development is going? The increasing attention to U-shaped performance that is reflected throughout is indicative of an increasing emphasis on the goal of explaining development. The time seems ripe for such renewed efforts at well-grounded developmental explanations and this issue illustrates the benefits that such efforts can produce.