This chapter documents the development of a new assessment instrument for vocabulary levels of bilingual kindergarten pupils in Developmental Education. It begins with a brief review of linguistic development in young children and the importance of contextual frameworks for the construction of lexical networks. The thematic approach to vocabulary expansion, which is based on Vygotskij's concept of the zone of proximal development and is foundational in Developmental Education provides such frameworks and results in effective vocabulary learning. Secondly the chapter critically discusses the standardised vocabulary test currently used in the Netherlands, paying specific attention to its ecological validity. Finally a thematic vocabulary assessment model based on meaningful and interactive communication is presented as a possible alternative to currently used standardised vocabulary tests in young children's classrooms. Research that examines the value of this approach empirically is briefly described.