Offered spring. Prereq., LING 270S or LING 470. Observing/describing language learners' behaviors and, to a degree, advances toward proficiency (i.e., fluency plus accuracy); the presence of error as conditioned by a priori knowledge of language and implications for child and adult development; and applying typical methods of linguistic analysis to the (non-) systematic variants in language form characterizing developmental processes as a way of trying to explain variable behavior.