The ability to apply and follow mathematical reasoning is an ability students should develop according to Curriculum for the compulsory school, preeschool class and the leisure- time centre 2011. The purpose of this survey was to find out how teachers in compulsory school and preeschool class work with the ability to apply and follow mathematical reasoning. I was also interested in investigating if participation in continuing professional development in mathematics influence the teachers way of working with the students. The survey consisted of six qualitative interviewees in which three of the interviewees were part of continuing professional development in didactics for teachers educating mathematics. The result showed that all interviewees consider that it is importent to communicate mathematics, both between students as well as between teachers and students. The teachers included in continuing professinonal development showed a clear consciousness in their work to reason and follow mathematical reasoning. However, among some interviewees the consiousness appeared to come from their experience and which reflects their way of working. The student's age and how far they have developed their language also appeared to be significance to how capable they are of reasoning. The teachers analyze the ability to apply and follow mathematical reasoning, it's meaning and usage, differently.