This chapter focuses on learners´ alternative conceptions. Since prospective teachers in general, and prospective computer science teachers in particular, face difficulties in gaining the notion of alternative conceptions, it is important to address this issue in the MTCS course and to deliver the message that a learning opportunity exists in each pupils´ mistake (or misunderstanding). Several pedagogical tools for exposing learners´ alternative conceptions are presented as well as three activities to be facilitated in the MTCS course.