In this paper we offer a theoretical discussion on the mathematical content knowledge development of secondary school mathematics teachers. We discuss relevant studies from literature and use different views and conceptions of school mathematics to propose a model to develop or transform the mathematical content knowledge of secondary school mathematics teachers in such a way that they are able to reconsider their beliefs about the nature of school mathematics. In the model we view mathematics teaching as a kind of problem solving where mathematical content knowledge has to be unpacked and communicated after teachers have reflected on the components of the model, namely different kinds of mathematical knowledge (procedural and conceptual), cognitive processes (problem solving, reasoning, communication, connections and representations) involved in doing mathematics, and working in different problem contexts.