Through recognising mathematics teachers as professionals who use mathematics in their workplace, this article traces a parallel between the mathematics enacted by teachers in their practice and the mathematics used in workplaces found in studies of professionals (e.g. nurses, engineers, bankers). This parallel is developed through the five characteristics highlighted in Noss' work (2002). We use this parallel to develop a conceptualisation of teachers' mathematics in their practice, and in turn to suggest implications concerning teachers' mathematical preparation.