The aim of this research was to examine the mechanisms behind children’s and adolescent’s motivation for learning an instrument. Self-determination theory (Deci & Ryan, 2017) served as the theoretical background: It assumes different forms of motivational regulations and organizes them on a continuum, ranging from autonomy to control. Satisfaction of the psychological basic needs for autonomy, competence and social relatedness support the development of autonomous forms of motivation. These Basic Needs are considered as mandatory not only with regard to SDT, they also play an essential role in the entire motivational research. Previous studies, particular in educational settings, mainly focused on teaching and learning in school, whereas learning outside of the school setting is rarely been taken into account (e.g. Evans, McPherson & Davidson, 2013). In order to explain motivation when learning an instrument, aside from the support of basic needs in music lessons, an autonomous parental educational style and the attitudes of the peer group towards learning and playing a musical instrument are investigated. Data from a survey study (N=856 music students from Austria) were analyzed using structural equation modeling (SEM). The results demonstrate that, in addition to the learning environment (support of the basic needs), the peer group and the parental educational style can also explain a significant variance of motivation. The proposed models show a significant effect for the parental educational style in explaining controlled motivation, whereas the attitudes of peers concerning learning an instrument and the music lessons were essential factors for explaining autonomous motivation.