In this article, we analyse the ways school headteachers/principals (HT/Ps) at different stages of adult ego development (AED) make sense of and respond to the organisational complexity of their schools. The AED stage of 20 HT/Ps in England was assessed using the Washington University Sentence Completion Test. HT/Ps in the self-aware, conscientious and individualist AED stage were identified. The research used a critical incident technique to analyse how principals made sense of and responded to the complexity of their organisations. Those HT/Ps in the self-aware, conscientious and individualist stages differed in the way they acknowledged complexity, their capacity to recognise and accept ambiguity and unpredictability; and the way they acknowledged emergence in resolving wicked problems. Importantly, the way these differences were different in individuals in the self-aware, conscientious and individualist stages showed a clear development trend with an increasing acknowledgement of complexity; a growing tolerance of ambiguity and unpredictability; and an increasing acknowledgement of emergence in complex contexts. The findings have implications for the way the role of HT/P is viewed and developed in actual and potential HT/Ps. One important implication is that the stage of HT/Ps' AED is likely to impact on their overall effectiveness in their role.