Language teacher cognition research is a growing field. In recent years several features of language teacher cognitions have been noted: they can be complex, ranging over a number of different subjects; they can be dynamic, changing over time and under different influences; and they can be systems, forming unified and cohesive personal or practical theories. However, as yet there is no single theoretical framework for studying language teacher cognitions. In this article I propose that complex systems theory might offer such a framework. I offer an exploratory investigation of the applicability of complex systems theory by focusing on the re-analysis of a previously published case study of the practical theory of an English language teacher teaching EFL in Armenia. I do this by discussing and presenting evidence of heterogeneity, dynamics, non-linearity, openness, and adaptation, which characterize complex systems, and are displayed by the EFL teacher's cognitions. I conclude by suggesting that complex systems theory is compatible with other lines of research, is able to be developed in field-specific ways, offers several lines of research as well as different methodological approaches, and has practical implications for language teacher development.