This paper argues for the use of "developmental" evaluation as a design-based research tool for sustainable curriculum innovation in professional higher education. Professional education is multi-faceted and complex with diverse views from researchers, professional practitioners, employers and the world of politics leaving little consensus about the nature of educational "problems", let alone educational "solutions". Developmental evaluation is an emerging approach to evaluating innovations and/or organisations that are in a continuous state of change; it asks the evaluators to not simply appraise a final design, but to work with designers through processes of rapid reconnaissance, mapping the territory and emergent modelling. This paper provides an account of how the adoption of the dispositions and approaches of developmental evaluation increased the trustworthiness of decision-making in the design of a new post-graduate teacher education degree in Australia. The principles of developmental evaluation as a design thinking approach have application in other complex curriculum settings.