Beliefs are known to influence learning processes and thus become relevant in the instruction of pre-service mathematics teachers with regard to the pedagogical content knowledge taught in courses of mathematics didactics at university. In exploring beliefs about mathematics didactics of pre-service teachers training for secondary school, 50 bachelor students (ca. 5th semester) responded to two open-ended tasks in which they were asked to express their understanding of mathematics didactics. With the help of qualitative content analysis, topics related to mathematics didactics as identified by the participants are categorized. The category system shows that beliefs of participants differ in some respects from what selected research associates with mathematics didactics. Also, technical aspects of lessons like designing lessons are frequently mentioned within the answers, whereas topics with regard to learners or curriculum are rarely addressed.