In this paper the author makes a short presentation of an ongoing longitudinal research project in Sweden. However, he wants to focus on the theoretical perspective he has found fruitful for his understanding of music teachers' professional socialisation, and on the question whether the music teacher must also be a musician in order to possess adequate subject competence. This is and has been a constantly current issue for presumptions about the design of the music teacher training program, the admission tests and the professional life as music teacher. The project is about a class of prospective music teachers in Sweden, from the time of their application tests for admission to the music teacher training program in 1988 up to their activity as professional music teachers in 1998. The German social philosopher and sociologist Jurgen Habermas' theory of communicative action is one of two themes the author proceeds from. The other theme is his view of society as divided in two parts, "lifeworld" and "system". With Habermas' concept of lifeworld as background, and the idea of communicative action as foreground, the author has developed understanding of the professional socialisation of the music teacher, bringing out and highlighting the complexity in this process. Through a "contextualisation," the complexity of the professional socialisation of music teachers has been possible to make clear. From the structural aspects of the lifeworld, transferred to voluntary music education as a professional context, there is substantial affinity with the music teacher's experience of his own pre-training and music teacher training. In the author's view, the question of what sort of training a music teacher should have is based on other qualities and competencies than solely those contained in issues of intra-musical judgement around music education as well as extra-musical criteria of judgement in teacher training. To him, it seems that such questions of judgement are blind-track; they lead nowhere. It is also here that he sees savoir-faire as a fundamental feature. It is possible that savoir-faire means that a role-identity as musician and a role identity as teacher can be contained within the same competence, on condition they are socially (contextually) situated and constructed. (Contains 2 figures and 1 footnote.)