The primary objective of this paper is to present a curriculum design model that extends the 'Powerful Knowledge' ideas of social realist theory. The Model called 'Curriculum Design Coherence' (CDC) hypothesizes an approach in which subject concepts and the subject's epistemic structure are central to the design process to enable deep learning. The model differentiates and then links subject concepts, subject content, subject competencies, and assessment. The underlying premise is that deep learning for students is more likely if teachers utilize and make visible the epistemic structure of the area of study; the subject concepts and subject competencies to be taught and their inter-relationships as 'knowledge-that' (epistemic knowledge) and 'knowledge-how-to' (procedural knowledge). The usefulness of the model for coherence in curriculum design is currently being tested by primary and secondary school teachers in New Zealand and England. The study takes a realist approach utilizing qualitative methods including workshops, analysis of curricular materials, and teacher interviews. The model is in the early stages of testing and some initial findings indicate its usefulness along with the challenges for teachers in engaging deeply with the structure of their subject.