This chapter surveys the implications of linguistic variation and diversity for language instruction. Sociolinguistic research amply documents the occurrence of regional and social diversity in all languages; variability is a universal property of human language. Everyone has implicit awareness of this in their native languages, and it needs focused attention in second language teaching and learning. It is a disservice to students to teach them a normative standard and neglect all else. Achieving communicative competence in a language requires some familiarity with dialect diversity, social and ethnic varieties, stylistic practices, and the social meaning of linguistic forms. It is important to teach basic facts about the social status of a language in the places it is spoken, and the presence of other languages: French is dominant in France, co-official with English in Canada, but mainly an L2 in ‘Francophone’ Africa; most Argentines are monolingual L1 Spanish speakers, but half of Bolivians speak indigenous languages as L1. Ongoing language change is important for learners to know about, both to comprehend the new forms, and to be aware of how they will be perceived.