In this plenary address, I suggest that Complexity Theory has the potential to contribute a transdisciplinary theme to applied linguistics. Transdisciplinary themes supersede disciplines and spur new kinds of creative activity (Halliday 2001 [1990]). Investigating complex systems requires researchers to pay attention to system dynamics. Since applied linguists study language systems that change (for example, as they develop in learners), this is a useful perspective to bring to bear on many of our concerns. To introduce Complexity Theory, I list twelve principles undergirding this perspective and elaborate on three of them: those to do with dynamism, complexity, and the role of context. I then discuss several studies of L2 development that have been informed by the perspective. I conclude by suggesting that the ultimate promise of Complexity Theory is the help it provides in humanizing science. (Contains 1 footnote.)