This article overviews current research on second language vocabulary learning. It concludes that a large vocabulary is necessary to function in English: 8000-9000 word families for reading, and perhaps as many as 5000-7000 families for oral discourse. In addition, a number of word knowledge aspects need to be learned about each lexical item. Taken together, this amounts to a substantial lexical learning challenge, one which many/most learners fail to meet. To facilitate adequate vocabulary learning, four vocabulary learning partners (students, teachers, materials writers, and researchers) need to contribute to the learning process. Vocabulary learning programs need to include both an explicit, intentional learning component and a component based around maximizing exposure and incidental learning. The four learning strands (meaning-focused input, meaning-focused output, language-focused learning, and fluency development) suggested by Nation (2001) provide a structure by which to integrate intentional and incidental vocabulary learning. The overriding principle for maximizing vocabulary learning is to increase the amount of engagement learners have with lexical items. All four learning partners need to acknowledge the incremental nature of vocabulary learning, and to develop learning programs which are principled, long-term, and which recognize the richness and scope of the lexical knowledge that needs to be mastered. (Contains 4 tables and 7 notes.)