The present study builds on teachers' professional knowledge about mathematics learning difficulties. Based on the input of 918 primary school teachers, an attempt is made to develop an overview of difficult curriculum topics in primary school mathematics. The research approach builds on new conceptions about the professional identity of teachers and earlier conceptions that point at the critical relevance of teachers' pedagogical content knowledge. It is also found that the adoption of a specific commercially available learning package (CALP: manuals and exercise books used in the classroom) plays a mediating role. (Contains 5 tables and 1 figure.)