This document summarizes key aspects of learner language and second language acquisition. It defines learner language as the written or spoken language produced by language learners. Interlanguage refers to a learner's developing second language system. Studying learner language helps teachers understand what is reasonable to expect and helps learners understand the acquisition process. Errors provide insight into a learner's progress and can indicate overgeneralization of rules. Developmental sequences show that learners acquire language in predictable stages, influenced somewhat by their first language but with many commonalities across learners. Grammatical features, questions, negation and other structures follow identifiable developmental paths in learner language.