The aim of this thesis is to illuminate which role theories of music education can have in the dialectic between discourse and competence in the practice of music pedagogy. The thesis stems from a problem-oriented project that resulted in the formulation of the following research question: how can the connection between music education theory and the power of definition be described and problematized from a cultural-historical perspective using Vygotsky's thesis of Semiotic Thinking? In his research on the dual function of language Vygotsky makes a distinction between the usage of concepts and conceptualization. In this study this distinction has been crucial to determining the relationship between music education theory and the power of definition as practised in music pedagogy. The research project comprised a background study and three sub-studies. The focus of the background-study was the conditions for musical learning and development in persons with severe cognitive dysfunctions. The first sub-study dealt with how social workers in this area of special education conceptualise their view of music in relation to their understanding of music activities in their own practice. The second sub-study aimed to explore which concepts music-therapists use in their meta-reflections on musical situations in this area of special education. The purpose of the third sub-study was to formulate a theoretical perspective on the relationship between music education theory and the power of definition using Vygotsky's hypothesis of semiotic thinking. The main conclusions are: first, that music education theory can be understood as a structural coupling between the power of definition that is embedded in music-pedagogical discourse as a whole, and the power of definition possessed by music pedagogues; second, that the development of the power of definition in the practice of music pedagogy is a creative process that in turn entails a critical questioning of music education theory.