This study examines the English and Spanish vocabulary skills that young Dual Language Learners (DLLs) bring to Head Start, as well as their vocabulary learning over the year. Further, we isolate the unique contributions of various child, family, teacher, and classroom factors to these skills. Participants were drawn from a recent cohort of the Head Start Family and Child Experiences Survey. Results show that, for both Spanish and English vocabulary, child and family factors, especially the prevalence of each language in the household, play a role in initial skills and end-of-year skills. The quality of the language of classroom instruction also predicts Spanish and English vocabulary learning over the year for all children; in English, this relation is significantly greater for children with the lowest initial skills. Findings elucidate potential leverage points for intervention to improve Spanish and English vocabulary outcomes during Head Start for these vulnerable early learners. (author abstract)