From a socio-constructivist perspective, learners’ engagement is of critical value for the implementation of multi-dimensional educational goals. So far, only few studies have empirically examined the links between the support and the effectiveness of different forms of learners’ engagement at commercial vocational schools. The thesis pursues I) to conceptualize and examine the learning potential of learners’ self-initiated contributions for the effectiveness of teaching and II) to identify and to analyse inﬂuencing factors of self-initiated participation. The ﬁndings of the mixed- methods approach show that deviating from traditional conversation scripts can o er learning chances for learners and teachers, because self-initiated participation reveals learners’ content gaps or misconceptions. In addition, self-initiated contributions containing or causing explanations to prove to be important for the effectiveness of teaching. However, the extent and the quality of self-initiated contributions in classroom situations di er considerably between different types of learners, teaching contexts and teacher reactions. This o ers important implications for teacher education: a) The advancement of teachers’ diagnostic competence to identify the learning-promoting potential of self-initiated contributions, b) the creation of learning contexts that support learning-promoting engagement, and c) the development of adaptive strategies for teachers to provide cognitive learning support.