With increasing immigration, bilingualism has become part and parcel of the everyday lives of many children. Although research indicates that under favourable circumstances bilingual children can become balanced bilinguals, especially immigrant children seem to have difficulty coping with the language of schooling. In the current thesis, 67 Turkish-Dutch immigrant children and 72 Dutch native speakers were followed during their kindergarten years and tested at 4, 5 and 6 years of age. For most Turkish immigrant children in the Netherlands, Turkish is their home language and school enrolment at the age of 4 marks their first submersion in a primarily Dutch language environment. The Turkish-Dutch children in our study did not catch-up with their Dutch peers in Dutch vocabulary level; their rate of development was even somewhat slower. As vocabulary at the start of kindergarten is an important predictor for later literacy development and overall school success, it is of urgent practical interest to disentangle different factors influencing bilingual vocabulary development from an early age. In the current thesis, it was shown that the cognitive factor referred to as verbal short-term memory is a crucial interface between language input and vocabulary acquisition not only in monolingual, but also in bilingual children. Although this factor has been suggested to be highly heritable, our results showed that it is influenced considerably by the language input children experience in their homes, and that all developmental improvements in the capacity of verbal short-term memory in the preschool period can be attributed to growing long-term knowledge support. Because Turkish-Dutch children receive less input in Dutch in their home environments due to the bilingual situation, they have less support from long-term knowledge of Dutch, and consequently more difficulty in remembering new words in that language, which constitutes a double risk. Especially knowledge of the statistical distribution of phoneme clusters in Dutch, ...