A major reading-achievement gap exists between English language learners and English-only students. In order for ELLs to experience school success, they must achieve English language proficiency. This article presents why vocabulary acquisition plays the most vital role in ELLs' learning of the English language. Factors include the severity and frequency of vocabulary errors, the disparity of vocabulary knowledge between ELLs and monolingual English speakers, and the high correlation between vocabulary and comprehension. This study is based on a four-component vocabulary program proposed by experts in the field and a synthesis of research findings, practical strategies, and tools that enhance ELLs' vocabulary development. Other critical considerations for an effective vocabulary program are also discussed. (Contains 6 tables.)