This document discusses second language acquisition. It defines second language acquisition as the process of learning a second language in addition to one's native language. It notes that second language acquisition refers to what the learner does, not what the teacher does. It also discusses some key concepts in second language acquisition, including interlanguage, which refers to a learner's language system with its own rules, and the typical sequence of acquisition of pronouns across languages, which follows a progression from omission to acquisition of person and then number and gender features.