The paper focuses on a priori analysis, one of the key concepts of the Theory of Didactical Situations. According to this theory a priori analysis is one of the main tools a teacher uses when planning a teaching unit.
This paper introduces a priori analysis in a different perspective - as a tool helping in planning problems, using them in lessons and in analyzing results in mathematics education research. Its importance for research is illustrated on an example from an ongoing research focusing on improvement of pupils' culture of problem solving.
Because of the limited scope of this paper, only one aspect of a priori analysis is introduced, namely problem solving strategies.