The traditional view of development is stage-like progress toward increasing complexity of form. However, the literature cites many examples in which children do worse before they do better. A major challenge for developmental theory, therefore, is to explain both global progress and apparent regression. In this article, we situate U-shaped development as a special case of the nonlinearity that is characteristic of all developmental process. We use dynamic systems theory to show how behavioral regression can be understood as part of the ordinary mechanisms of change. Examples from our work in infant motor and language development illustrate the ways that U-shaped behavior arises from continuous changes in the collective dynamics of multiple, contingent processes. A central claim is that true regression is not possible because behavior exists continuously in time. Thus the current state of the system always depends on its past history. Instead, the appearance of regression reflects the concept of softly assembled behavior, or the ability of contributing components to self-organize in different configurations that depend upon the status of the components, the environment, and the task. (Contains 5 figures.)