We outline the nature of dynamic systems, both linear and nonlinear, and we review dynamic systems principles that apply well to various aspects of human development, including the emergence of new forms, phases of stability and instability, continuous and discontinuous change, and differentiation among individual trajectories. We then document the growth and popularity of dynamic systems approaches for understanding a diverse array of developmental domains. Finally, we acknowledge the contribution and impact Esther Thelen has made in this area of developmental science, and dedicate this special issue to her memory.