This paper describes pre-service teachers’ conceptions and beliefs regarding reasoning and proof in teaching at the primary level before an intervention targeting their ability to design teaching mathematics aimed at reasoning and proof. Group interviews were conducted and analysed. It transpired that future lower and upper-secondary mathematics teachers see reasoning and proof as an important part of mathematics education. Yet, they have different understandings of the notion of reasoning. The pre-service teachers also shared some worries connected to teaching reasoning and proof. These concerned mainly time limitations, the need to adjust teaching because of high-stake examinations or difficulty in engaging most pupils in the classroom with what they consider demanding tasks. Results serve to fine-tune the intervention and to observe what changes the intervention caused.