Two empirical studies set out to explore the relation between breadth and depth of word knowledge and to link these concepts with language acquisition and frequency of language input. In the first study, the breadth and depth of word knowledge of 50 Dutch monolingual and bilingual kindergartners were investigated using receptive vocabulary, description, and association tasks. The second study examined the relation between the probability of knowing a word and the input frequency of that word in 1,600 Dutch monolingual and bilingual 4- and 7-year-olds. These studies found that there was no conceptual distinction between breadth and depth of vocabulary, and that breadth and depth were affected by the same factors for both monolingual and bilingual speakers. Very high correlations were found between monolingual and bilingual speakers with respect to the probability of knowing a word, which was strongly related to the input frequency in primary education.