This article surveys the development of second language (L2) phonology over the last 40-50 years. Research in this area has grown from analyzing learners' errors in terms of Contrastive Analysis to proposals explaining L2 sound patterns in terms of constraints on interlanguage grammar. Although native language transfer has endured as one source of learner constraints, researchers have, over the years, shown the necessity of phonological universals in explaining L2 phonologies. More recently, L2 phonologists have provided insightful analyses through constraint-based grammars within the framework of Optimality Theory. (Contains 1 footnote.)