There are different epistemologies that take part into the didactic action; for this reason an aware and significant epistemological training plays an extraordinary role in teachers’ training. Concepts like milieu and obstacles (epistemological and didactical) work out to be useful for such training. In this work we deal with this issue and we provide significant examples that show how a defective epistemological preparation negatively influences the classroom action. In this ambit, the didactical contract is also taken into account.