The author discusses the balance and mutual influence of the language of instruction and mathematics in the context of CLIL, Content and Language Integrated Learning. Different aspects of the relationship of language and Mathematics teaching and learning are discussed: the benefits of using a foreign language of instruction, as well as the advantages of using Mathematics as a tool to teach a foreign language. Based on research among pre-service teacher trainees, the author presents different approaches to CLIL-lesson planning, pointing out the focus of the teacher trainees on content and/or language and the development throughout the ongoing course of CLIL at the University, with respect to the different specializations of the teacher trainees.