It goes without saying that in Japan the foreign language classroom is often the only chance for the students to interact in or even hear the language they are learning. This goes in particular for languages like German, which hardly ever appear in the students' everyday lives. Consequently, the language the teacher uses in the classroom (instruction medium) plays a vital role for the students' learning process. Previous research has shown that according to different functions of the classroom, for example giving instructions or feedback, explaining vocabulary items, disciplining students, providing background information on the target language or culture etc. either the students' L1 or their target language is more appropriate. The article outlined below provides a survey of theoretical findings and research results concerning the use of the instruction medium in foreign language teaching. It will also be argued that German teachers in Japan need to become more aware of how they use the students' mother tongue and the target language (German) in their classrooms. Therefore, an action research plan has been devised for the purpose of investigating and improving the use of the instruction medium in the German classroom in Japan.