In this article, we describe the development and trial of a bilingual computerized test of vocabulary size, the number of words the learner knows, and strength, a combination of four aspects of knowledge of meaning that are assumed to constitute a hierarchy of difficulty: passive recognition easiest, active recognition, passive recall, and active recall hardest. The participants were 435 learners of English as a second language. We investigated whether the above hierarchy was valid and which strength modality correlated best with classroom language performance. Results showed that the hypothesized hierarchy was present at all word frequency levels, that passive recall was the best predictor of classroom language performance, and that growth in vocabulary knowledge was different for the different strength modalities.