This chapter considers the application of the scientist-practitioner model in the area of coaching psychology. There is growing interest in coaching as a form of psychological intervention aimed primarily at non-clinical populations. The scientist-practitioner model is presented as an important differentiator between the coaches who practise from a background in professional psychology, and coaches whose experience is not informed by formal training in the behavioural sciences. After describing coaching psychology and then briefly considering the main theoretical models we employ, we critically assess the strengths and limits of the scientist-practitioner model in the light of complexity theory. Complexity theory provides a useful description of the iterative and organic nature of psychological interventions, and provides a corrective to a linear reductionist understanding of the scientist-practitioner model. Rather than seeing the role of science and practice as developing prescriptive models of psychological practice, we argue for an evidence-based approach to coaching psychology. This approach values the contribution of both science and practice in the process of informing coaching interventions.