Reflection in action research is a complex matter, as is action research itself. In recent years, complexity science has regularly been called upon in order to more thoroughly understand the complexity of action research. The present article investigates the benefits that complexity science may yield for reflection in action research. This article begins by explicating the sense in which the complexity of reflection in action research involves the role of values and existential knowledge in education. The gap between theory and practice is also explored. On the basis of a number of common features of complex systems (heterogeneous, open, dynamic, non-linear, adaptive, and co-adaptive), the sense in which reflection can be regarded as a complex system is discussed. To this end, the features of complex systems are translated into features of reflection in action research, which, in turn, are illustrated with examples from recent publications on reflection in action research. The aim of this analysis is to make reflection in action research more understandable and manageable. In line with this, it is argued that room for insecurity and unpredictability, combined with an explicit consideration of reflection as a complex system, contributes to the use of complexity as a stimulus for new learning.