This article discusses the impact of lexical range on the learners' ability to communicate in English when taught as a foreign language in bilingual schools, and emphasizes the importance of explicit vocabulary instruction. It draws on data from classroom observation, lexis-retrieval tasks, written and spoken performance in bilingual (German-English) and regular school classes at grades 5-8 in Austrian secondary schools. Results suggest that a wider lexical range results in better communicative competence and fluency and that breakdown of communication in spoken or written performance is more frequently caused by insufficient vocabulary rather than by lack of control or grammatical problems. Consequently, insights from cognitive linguistics, the Lexical Approach, and Lexical Priming are discussed to justify the concept of vocabulary instruction in contextualized units. Taking the scarceness of theoretical and practical concepts into consideration, the need for research on the explicit instruction of vocabulary and metacognitive strategies is fleshed out.