Based on a theoretical model of instructional quality this study investigates the influence of different dimensions of instructional quality in the school entrance phase on students' language development taking into account child and family background factors as well as structural characteristics of the classroom and teacher's educational beliefs. The vocabulary skills of a German sample of 554 children in grade 2 was analysed using multivariate regression models to test the extent to which instructional quality is related to the development of vocabulary in grade 2. Additionally, we examined whether structural characteristics and teachers' educational beliefs were related to vocabulary development. Results show positive relationships between global dimensions of instructional quality and students' vocabulary development. Furthermore, teachers' educational beliefs are also relevant for vocabulary development, whereas structural characteristics are subordinated. Thus, attention should be paid to the role of high-quality teaching practices and teacher beliefs.