This paper considers a number of diverse contexts in which English is learned as a second language and in which nonstandard dialects arise because of social and linguistic factors. The varieties considered here are immigrant English, indigenous-minority varieties of English, pidginization and creolization, local varieties of non-native English, and English as a foreign language as a branch of study. The learning processes and dialects are discussed in terms of interlanguage, seen as the learner's approximate system, that is, the intermediate stage between the source and target language which results from transfer, transfer of training, strategies of communication, learning, and overgeneralization. The concept of interlanguage provides a basis for dialect and language variety description, because it considers rules which are linguistic in origin -- derivable from the mother tongue and limited exposure to the target language -- and social in origin -- derived from communication and learning strategies. Implications of the interlanguage theory in terms of learning English as a foreign language are also discussed. (VM)