Second language development can be viewed as a complex and dynamic process in which learners follow non-linear trajectories and develop their language over a period of time (Larsen-Freeman, 2006). Intrinsic to the view of Complex Dynamic Systems Theory (CDST), a system is composed of hierarchical, interdependent subcomponents (learner, learner language, and environment) in dynamic relations as learners develop their interlanguage over time. This piece reviews the interactive and dynamic nature of the subcomponents illuminating the transitory nature or variability (in patterns of language development) that is inherently a characteristic of a complex dynamic system (Larsen-Freeman & Cameron, 2008). First, the forum piece describes how the growth of a subsystem can affect other parts of the system, followed by an explanation of the diverse interrelationships between the connecting subsystems. Next, three empirical studies on the varied relationships between the subsystems are presented. The article concludes with a critique of the current state of research on this topic, revealing some gaps that need to be addressed in future research.