This paper makes a case for placing knowledge at the centre of the school mathematics curriculum, and for knowledge building and knowledge differentiation as critical for both equity and excellence, emphasising that knowledge is much more than a set of descriptions of content as might typically be found in a curriculum document or textbook. The paper commences by discussing the implications of the traditional epistemological view of knowledge as justified true belief for mathematics education and uses this to build a preliminary description of knowledge building. Ideas from critical realism are then used to show that it is not so much the content of knowledge that matters but the production of knowledge and to build an enhanced conception of knowledge building in school mathematics. A distinction is made between knowledge and knowing that provides a non-relativist yet fallible view of knowledge, recognising its emergent but directed nature through its production and legitimation within established fields. The importance of knowledge building as a democratic right is then discussed, highlighting the importance of specialised knowledge and arguing that knowledge differentiation provides a basis for a conception of school mathematics curriculum that is dynamic and empowering. The paper concludes by discussing a range of potential theoretical and empirical research projects arising from a focus on knowledge and knowledge building in school mathematics.