Linked research and development forms the central pillar of the 5-year Wits Maths Connect Secondary Project in South Africa. Our empirical data emphasised the need for teaching that mediates towards mathematics viewed as a network of scientific concepts, and the development of the notion of 'mathematical discourse in instruction' (MDI), as an analytic tool and discursive resource for working on research and professional development. This paper describes and reflects on MDI, its emergence in a particular education context, and what this discursive resource offers more generally as it works across different discourses and practices. [This work is based on research of the Wits Mathematics Connect Project, University of the Witwatersrand, supported by Department of Science and Technology, and FirstRand Foundation, and administered by the National Research Foundation.]