The guiding philosophy of this theoretical work lays in the argument that mathematics teachers' professional knowledge is the integration of various knowledge facets derived from different sources including teaching experience and research. This paper goes beyond past trends identifying what the teachers' knowledge is about (content) by providing new perspectives, in particular, on how the knowledge is structured and organised (form), on what teachers' draw on their knowledge (source), and whether the knowledge is stable and coherent or contextually-sensitive and fluid (nature).