The objective of this study is to demonstrate that the criteria of didactical suitability, proposed by the theoretical framework known as the Onto-Semiotic Approach (OSA) of mathematical knowledge and instruction, are powerful tools for organizing the reflection and assessment of instruction processes carried out by mathematics teachers. To this aim, the results of a multiple case study are presented which prove that when teachers are faced with the task of evaluating processes of instruction, they employ--either implicitly or explicitly--the OSA criteria of didactical suitability (epistemic, cognitive, affective, mediational, interactional and ecological). Therefore, the explicit application of said criteria in teachers' education cycles contributes to the development of the didactic analysis competence that is necessary for teachers to be able to reflect on their own practices.