Despite acknowledging the complex nature of vocabulary knowledge, researchers have rarely investigated the dimensionality of this construct empirically. This study was designed to test a multi-dimensional model of English vocabulary knowledge for sixth-grade students from linguistically diverse backgrounds (n = 584). Participants included language minority students learning English as a second language (L2) and students who learned English as a first language (L1). Students were assessed on 13 reading-based measures tapping various aspects of vocabulary knowledge. Using multiple-group confirmatory factor analysis, we found that vocabulary was comprised of three highly related, but distinct dimensions--breadth, contextual sensitivity, and morphological awareness. This three-dimensional model was found to hold for L2 learners as well as L1 speakers. Although the L2 learners were statistically significantly lower than the L1 students on all three dimensions, the magnitude of the difference for morphological awareness (d = 0.37) was somewhat smaller than that for vocabulary breadth (d = 0.52) and contextual sensitivity (d = 0.49). Results were similar for a subsample of Spanish-speaking L2 learners and for the full sample of L2 learners from various home language groups. Findings support a distinction between word-specific and word-general knowledge in understanding individual and group differences in vocabulary.