The articles featured in this issue make apparent the variety of perceptual and cognitive competencies that follow curvilinear developmental courses as well as the complexities inherent in accounting for such phenomena. What is revealed is the way in which a fit is achieved between organisms and the environments they occupy. Curvilinear development unveils the process by which early knowledge is transformed into more complex, stable, and mature forms through the intricate dance between organism and environment across developmental time. In this paper, the author considers four key issues germane to the implications of curvilinear trajectories for cognitive development from the stance of the articles in this volume: (a) is curvilinear development more apparent than real?; (b) is curvilinear development continuous?; (c) if so, what is it that continues?; and finally, (d) what propels a developing system into periods of non-monotonic change? Then, as an extension of the answer to this final question, the author returns to a wide-angle view to consider the functional significance of recurring non-monotonic change over the course of ontogeny. (Contains 1 figure.)