This Element explores the roles of explicit and implicit learning in second language acquisition. The authors lay out some key issues that they take to underlie the debate on the extent to which second language acquisition involves explicit learning, implicit learning, or both. They also discuss what they take to be an oversight in the field: namely, the lack of clear definitions of key constructs. Taking a generative perspective on the nature of language, while addressing alternative approaches at key points, they refocus the discussion of explicit and implicit learning by first asking what must be learned (i.e., what is this mental representation we call "language" that all functioning humans possess?) The discussion and research reviewed leads to the conclusion that second language acquisition is largely if not exclusively implicit in nature and that explicit learning plays a secondary role in how learners grapple with meaning.