Over the last decades, there has been an on-going international reform for school mathematics, which has, not surprisingly, been difficult to implement. This study focuses on teachers’ interpretation of formal written curriculum documents, especially whether their interpretations align with how a concept (the concept of problem) is conveyed in the documents (in Sweden). The results show that the formal written documents are vague, but that it to some extent conveys the concept of problem as “a task for which the solution method is not known in advance to the solver.” The interviews show that about 53 % of the teachers interpreted problem as “any task,” and that teachers’ interpretationstherefore are not aligned with how the concept is (albeit vaguely) conveyed in the documents.