The way in which music teachers choose and use the subject content in ensemble and music theory in the upper secondary school is focused in this study. The point of interest is the everyday classroom teaching of music teachers. The intentions that music teachers have with their teaching is also studied. This thesis is a study of music teaching and is a subject-didactic investigation, inspired by variation theory. The overarching aim of the thesis is to study how music teachers in upper secondary school choose teaching content when teaching ensemble and music theory. The teachers’ use of the teaching content implies that they choose to focus and teach certain parts in their teaching. This choice of con- tent and how it is used is in focus in this study. The research questions are: How do music teachers choose teaching content when teaching ensemble and music theory in upper secondary school? How do music teachers use the teaching con- tent in their teaching? The data collection includes video-documented lessons and qualitative inter- views with five music teachers in upper secondary school in 2004. The analysis reveals two different choices of content. When the teachers have music and theory as the content of their teaching, it is the content that guides the teaching methods, where the teacher uses a fixed content, which is then presented in different ways; this was mainly in music theory. When the content is music activities, the music teachers adjust the activity-based content in accordance with the level of the pupils’ skills in ensemble. The differences that occur in the varia- tion theory results are closely related to the teaching content. The foundations of the two different choices of content that are made by the music teachers, as well as the significance of the learning objects, are finally discussed.